Time division partial encryption

ABSTRACT

An encryption arrangement for multiple encryption of television programs. A system according to embodiments of the present invention multiple encrypts only a portion of the data required for full presentation of a television program to permit coexistence of multiple conditional access encryption systems associated with multiple manufacturer&#39;s set-top boxes within a single system. In one embodiment, a time slicing technique is used to encrypt portions of a program on a time allocated basis. By only encrypting a portion of the program, dramatically less bandwidth is consumed than the alternative of multiple encryption of all program data, thus permitting a larger number of programs to be carried over the same bandwidth while permitting coexistence of multiple conditional access systems in a single cable television system.

CROSS REFERENCE TO RELATED DOCUMENTS

[0001] This application is related to U.S. provisional patentapplication serial number 60/296,673 filed Jun. 6, 2001 to Candelore, etal. entitled “Method for Allowing Multiple CA Providers to Interoperatein a Content Delivery System by Sending Video in the Clear for SomeContent, and Dual Carriage of Audio and Dual Carriage of Video and Audiofor Other Content”, and provisional patent application serial number60/304,241 filed Jul. 10, 2001 to Unger et al., entitled “IndependentSelective Encryptions of Program Content for Dual Carriage”, andprovisional patent application serial number 60/304,131 filed Jul. 10,2001 to Candelore et al., entitled “Method for Allowing Multiple CAProviders to Interoperate in a Content Delivery System by PartialScrambling Content on a Time Slice Basis” and to U.S. provisional patentapplication serial no. 60/______, filed on Oct. 26, 2001 to Candelore etal., entitled “Television Encryption Systems”, docket number SNY-R4646P,which are hereby incorporated herein by reference.

[0002] This application is being filed simultaneously with patentapplications docket number SNY-R4646.01 entitled “Critical PacketPartial Encryption” to Unger et al., serial number ______; docket numberSNY-R4646.03 entitled “Elementary Stream Partial Encryption” toCandelore, serial number ______; docket number SNY-R4646.04 entitled“Partial Encryption and PID Mapping” to Unger et al., serial number______; and docket number SNY-R4646.05 entitled “Decoding and Decryptingof Partially Encrypted Information” to Unger et al., serial number______. These simultaneously filed patent applications are herebyincorporated by reference herein.

COPYRIGHT NOTICE

[0003] A portion of the disclosure of this patent document containsmaterial which is subject to copyright protection. The copyright ownerhas no objection to the facsimile reproduction of the patent document orthe patent disclosure, as it appears in the Patent and Trademark Officepatent file or records, but otherwise reserves all copyright rightswhatsoever.

FIELD OF THE INVENTION

[0004] This invention relates generally to the field of encryptionsystems. More particularly, this invention relates to systems, methodsand apparatus for providing partial encryption and decryption of digitalof television signals.

BACKGROUND OF THE INVENTION

[0005] Television is used to deliver entertainment and education toviewers. The source material (audio, video, etc.) is multiplexed into acombined signal which is then used to modulate a carrier. This carrieris commonly known as a channel. (A typical channel can carry one analogprogram, one or two high definition (HD) digital program(s), or several(e.g. nine) standard definition digital programs.) In a terrestrialsystem, these channels correspond to government assigned frequenciesfrequencies and are distributed over the air. The program is deliveredto a receiver that has a tuner that pulls the signal from the air anddelivers it to a demodulator, which in turn provides video to a displayand audio to speakers. In a cable system the modulated channels arecarried over a cable. There may also be an in-band or out-of-band feedof a program guide indicating what programs are available and theassociated tuning information. The number of cable channels is finiteand limited by equipment/cable bandwidth. Cable distribution systemsrequire a significant capital investment and are expensive to upgrade.

[0006] Much of television content is valuable to its producers,therefore copyright holders want to control access and restrict copies.Examples of typically protected material include feature films, sportingevents, and adult programming. Conditional access (CA) systems are usedto control availability of programming in content delivery systems suchas cable systems. CA systems come as matched sets—one part is integratedinto the cable system headend and encrypts premium content, the otherpart provides decryption and is built into the set-top boxes (STB)installed in user's homes. Several CA systems are used in the cableindustry including those provided by NDS (Newport Beach, Calif.),Motorola (Schaumberg, Ill.) and Scientific Atlanta (Atlanta, Ga.). Thismatched set aspect of CA systems has the effect that the “legacy” vendoris locked in as the supplier of additional STBs. Since the varioustechnologies for conditional access are not mutually compatible (and areoften proprietary), any new potential supplier is forced to license thelegacy CA. Thus, the cable operator finds itself unable to acquire newertechnology or competing technology from other set-top box manufacturerssince the technology owners are often unwilling to cooperate, or chargereasonable license fees. This inflexibility can be especiallytroublesome when cable companies with disparate CA systems are merged.Service providers would like more than one source for STBs for anynumber of reasons.

[0007] Once a cable operator picks an encryption scheme, it is difficultto change or upgrade the content encryption scheme without introducing abackward compatible decoding device (e.g. set-top box). Providingmultiple mode capability in new set-top boxes to handle multipleencryption systems can add substantial cost to any new set-top box,providing that the technology can be made available to the STB vendor toprovide the multiple decryption capability.

[0008] The only known current option to avoiding domination by thelegacy vendor (short of wholesale replacement) is using “full dualcarriage”. Full dual carriage means that transmission is duplicated foreach encrypted program—once for each type of CA encryption to be used.To provide full dual carriage, the headend is enhanced to provide eachform of CA simultaneously. Legacy STBs should not be impacted and shouldcontinue to perform their function despite any change. However, fulldual carriage often comes at an unpalatable price because of thebandwidth impact, thus reducing the number of unique programs available.Generally, the number of premium channels suffers so that the number ofoptions available to the viewer are limited and the value that can beprovided by the cable operator is restricted.

[0009] A conventional cable system arrangement is depicted in FIG. 1. Insuch a system, the cable operator processes audio/video (A/V) content 14with CA technology from manufacturer A (system A) using CA encryptionequipment 18 compliant with system A at the cable system—headend 22. Theencrypted A/V content along with system information (SI) 26 and programspecific information (PSI) 27 is multiplexed together and transmittedover the cable system 32 to a user's STB 36. STB 36 incorporatesdecrypting CA equipment from system A (manufacturer A) 40 that decryptsthe A/V content. The decrypted A/V content can then be supplied to atelevision set 44 for viewing by the user.

[0010] In a cable system such as that of FIG. 1, digital program streamsare broken into packets for transmission. Packets for each component ofa program (video, audio, auxiliary data, etc.) are tagged with a packetidentifier or PID. These packet streams for each component of allprograms carried within a channel are aggregated into one compositestream. Additional packets are also included to provide decryption keysand other overhead information. Otherwise unused bandwidth is filledwith null packets. Bandwidth budgets are usually adjusted to utilizeabout 95% of the available channel bandwidth.

[0011] Overhead information usually includes guide data describing whatprograms are available and how to locate the associated channels andcomponents. This guide data is also known as system information or SI.SI may be delivered to the STB in-band (part of the data encoded withina channel) or out-of-band (using a special channel dedicated to thepurpose). Electronically delivered SI may be partially duplicated inmore traditional forms—grids published in newspapers and magazines.

[0012] In order for a viewer to have a satisfying television experience,it is generally desirable that the viewer have clear access to bothaudio and video content. Some analog cable systems have used variousfiltering techniques to obscure the video to prevent an unauthorizedviewer from receiving programming that has not been paid for. In such asystem, the analog audio is sometimes sent in the clear. In the MotorolaVideoCipher 2 Plus system used in C-band satellite transmissions, strongdigital audio encryption is used in conjunction with a relatively weakprotection of the analog video (using sync inversion). In airlinein-flight movie systems, the availability of audio only through rentalof headphones has been used to provide the full audio and video only topaying customers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The features of the invention believed to be novel are set forthwith particularity in the appended claims. The invention itself however,both as to organization and method of operation, together with objectsand advantages thereof, may be best understood by reference to thefollowing detailed description of the invention, which describes certainexemplary embodiments of the invention, taken in conjunction with theaccompanying drawings in which:

[0014]FIG. 1 is a block diagram of a conventional conditional accesscable system.

[0015]FIG. 2 is a block diagram of a system consistent with oneembodiment of the present invention in which dual encrypted audio istransmitted along with clear video.

[0016]FIG. 3 is a block diagram of a system consistent with anembodiment of the present invention in which portions of programming aredual encrypted according to a time slice mechanism.

[0017]FIG. 4 is a flow chart of a dual encryption process consistentwith certain embodiments of the present invention.

[0018]FIG. 5 is a flow chart of a decryption process consistent withcertain embodiments of the present invention.

[0019]FIG. 6 is a block diagram of a system consistent with anembodiment of the present invention in which portions of programming aredual encrypted on a packet basis.

[0020]FIG. 7 is a flow chart of a dual encryption process consistentwith certain embodiments of the present invention.

[0021]FIG. 8 is a flow chart of a decryption process consistent withcertain embodiments of the present invention.

[0022]FIG. 9 is a block diagram of a system consistent with anembodiment of the present invention in which system information isencrypted and programming is sent in the clear.

[0023]FIG. 10 is a block diagram of a generic system consistent withvarious embodiments of the present invention.

[0024]FIG. 11 is a block diagram of a first embodiment of implementationof an encryption system consistent with embodiments of the presentinvention in a cable system headend.

[0025]FIG. 12 is a block diagram of a second embodiment ofimplementation of an encryption system consistent with embodiments ofthe present invention in a cable system headend.

[0026]FIG. 13 is a flow chart of an overall encryption process used toimplement certain embodiments of the present invention in a cable systemheadend.

[0027]FIG. 14 is a block diagram of a first embodiment of a set-top boximplementation of a decoding system consistent with embodiments of thepresent invention.

[0028]FIG. 15 is a block diagram of a second embodiment ofimplementation of a decoding system consistent with embodiments of thepresent invention in a cable system STB.

[0029]FIG. 16 is a block diagram of a third embodiment of implementationof a decoding system consistent with embodiments of the presentinvention in a cable system STB.

[0030]FIG. 17 illustrates the PID remapping process carried out in oneembodiment of a set-top box PID re-mapper.

[0031]FIG. 18 is a block diagram of an exemplary decoder chip that canbe utilized in a television set-top box consistent with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0032] While this invention is susceptible of embodiment in manydifferent forms, there is shown in the drawings and will herein bedescribed in detail specific embodiments, with the understanding thatthe present disclosure is to be considered as an example of theprinciples of the invention and not intended to limit the invention tothe specific embodiments shown and described. In the description below,like reference numerals are used to describe the same, similar orcorresponding parts in the several views of the drawings. The terms“scramble” and “encrypt” and variations thereof are used synonymouslyherein. Also, the term “television program” and similar terms can beinterpreted in the normal conversational sense, as well as a meaningwherein the term means any segment of A/V content that can be displayedon a television set or similar monitor device.

Overview

[0033] Modern digital cable networks generally use CA systems that fullyencrypt digital audio and video to make programming inaccessible exceptto those who have properly subscribed. Such encryption is designed tothwart hackers and non-subscribers from receiving programming that hasnot been paid for. However, as cable operators wish to provide theirsubscribers with set-top boxes from any of several manufacturers, theyare frustrated by the need to transmit multiple copies of a singleprogram encrypted with multiple encryption technologies compliant withthe CA systems of each STB manufacturer.

[0034] This need to carry multiple copies of the programming (called“full dual carriage”) uses up valuable bandwidth that could be used toprovide the viewer with additional programming content. Certainembodiments of the present invention address this problem in which thebandwidth requirements to provide an equivalent to multiple carriage areminimized. The result could be described as “Virtual Dual Carriage”since the benefits of full dual carriage are provided without the fullbandwidth cost. Several embodiments of the present invention arepresented herein to accomplish effective partial scrambling. Theseembodiments vary by the criteria used to select the portion to encrypt.The portion selected in turn affects the additional bandwidthrequirements and the effectiveness of the encryption. It may bedesirable to use one encryption process or several processes incombination in a manner consistent with embodiments of the presentinvention.

[0035] Certain of the implementations of partial dual encryptiondescribed herein utilize an additional (secondary) PID for eachduplicated component. These secondary PIDs are used to tag packets thatcarry duplicated content with an additional encryption method. The PSIis enhanced to convey information about the existence these new PIDs insuch a way that inserted PIDs are ignored by legacy STBs but can beeasily extracted by new STBs.

[0036] Some implementations of partial dual encryption involveduplicating only certain packets tagged with a given PID. Methods forselecting which packets to encrypt are detailed hereinafter. Theoriginal (i.e. legacy) PID continues to tag the packets encrypted withlegacy encryption as well as other packets sent in the clear. The newPID is used to tag packets encrypted by the second encryption method.Packets with the secondary PID shadow the encrypted packets tagged withthe primary PID. The packets making up the encrypted pairs can occur ineither order but, in the preferred implementation, maintain sequencewith the clear portion of the PID stream. By use of the primary andsecondary PIDs, the decoder located in the set-top box can readilydetermine which packets are to be decrypted using the decryption methodassociated with that set-top box, as will be clear upon consideration ofthe following description. The processes used to manipulate PIDs will bedescribed later in greater detail.

[0037] The encryption techniques described herein can be broadlycategorized (according to one categorization) into three basicvariations—encrypting just a major portion (i.e. audio), encrypting justthe SI, and encrypting just selected packets. In general, each of theencryption techniques used in the embodiments disclosed herein seek toencrypt portions of the an A/V signal or associated information whileleaving other portions of the A/V signal in the clear to conservebandwidth. Bandwidth can be conserved because the same clear portion canbe sent to all varieties of set-top boxes. Various methods are used toselect the portions of information to be encrypted. By so doing, thevarious embodiments of this invention eliminate the traditional“brute-force” technique of encrypting the entire content in one specificscrambling scheme, which predicates the redundant use of bandwidth ifalternate scrambling schemes are desired. In addition, each of thepartial dual encryption schemes described herein can be used as a singlepartial encryption scheme without departing from embodiments of thepresent invention.

[0038] The various embodiments of the invention use several processes,alone or in combination, to send substantial portions of content in theclear while encrypting only a small amount of information required tocorrectly reproduce the content. Therefore the amount of informationtransmitted that is uniquely encrypted in a particular scrambling schemeis a small percentage of the content, as opposed to the entirereplication of each desired program stream. For purposes of theexemplary systems in this document, encryption system A will beconsidered the legacy system throughout. Each of the several encryptiontechniques described above will now be described in detail.

[0039] The various embodiments of the invention allow each participatingCA system to be operated independently. Each is orthogonal to the other.Key sharing in the headend is not required since each system encryptsits own patents. Different key epochs may be used by each CA system. Forexample, packets encrypted with Motorola's proprietary encryption canuse fast changing encryption keys using the embedded security ASIC,while packets encrypted with NDS′ smart card based system use slightlyslower changing keys. This embodiment works equally well for ScientificAtlanta and Motorola legacy encryption.

Encrypted Elementary Stream

[0040] Turning now to FIG. 2, one embodiment of a system that reducesthe need for additional bandwidth to provide multiple carriage isillustrated as system 100. In this embodiment, the system takesadvantage of the fact that viewing television programming without audiois usually undesirable. While there are exceptions (e.g., adultprogramming, some sporting events, etc.), the typical viewer is unlikelyto accept routine viewing of television programming without being ableto hear the audio. Thus, at headend 122, the video signal 104 isprovided in the clear (unencrypted) while the clear audio 106 isprovided to multiple CA systems for broadcast over the cable network. Inthe exemplary system 100, clear audio 106 is provided to an encryptionsystem 118 that encrypts audio data using encryption system A(encryption system A will be considered the legacy system throughoutthis document). Simultaneously, clear audio 106 is provided toencryption system 124 that encrypts the audio data using encryptionsystem B. Clear video is then multiplexed along with encrypted audiofrom 118 (Audio A) and encrypted audio from 124 (Audio B), systeminformation 128 and program specific information 129.

[0041] After distribution through the cable system 32, the video, systeminformation, program specific information, Audio A and Audio B are alldelivered to set-top boxes 36 and 136. At legacy STB 36, the video isdisplayed and the encrypted audio is decrypted at CA system A 40 forplay on television set 44. Similarly, at new STB 136, the video isdisplayed and the encrypted audio is decrypted at CA system B 140 forplay on television set 144.

[0042] Audio has a relatively low bandwidth requirement compared with acomplete A/V program (or even just the video portion). The currentmaximum bit rate for stereophonic audio at 384 Kb/second isapproximately 10% of a 3.8 Mb/second television program. Thus, for dualcarriage of only encrypted audio (with video transmitted in the clear)in a system with ten channels carried with 256 QAM (quadrature amplitudemodulation), a loss of only about one channel worth of bandwidth wouldoccur. Therefore, approximately nine channels could be carried. This isa dramatic improvement over the need to dual encrypt all channels, whichwould result in a decrease in available channels from ten to five. Wheredeemed necessary, e.g., sporting events, pay per view, adultprogramming, etc., dual encryption of both audio and video can still becarried out, if desired.

[0043] Both legacy and new set-top boxes can function in a normal mannerreceiving video in the clear and decrypting the audio in the same mannerused for fully decrypting encrypted A/V content. If the user has notsubscribed to the programming encrypted according to the above scheme,at best the user can only view the video without an ability to hear theaudio. For enhanced security over the video, it possible to employ otherembodiments of the invention (as will be described later) here as well.(For example, the SI may be scrambled to make it more difficult for anon-authorized set-top box to tune to the video portion of the program.)Unauthorized set-top boxes that have not been modified by a hacker, willblank the video as a result of receipt of the encrypted audio.

[0044] Authorized set-top boxes receive Entitlement Control Messages(ECM) that are used to get access criteria and descrambling keys. Theset-top box attempts to apply the keys to video as well as the audio.Since the video is not scrambled, it simply passes through the set-topboxes' descrambler unaffected. The set-top boxes do not care that thevideo is in-the-clear. The un-modified and un-subscribed set-top boxesbehave as being un-authorized for the scrambled audio as well as theclear video. The video, as well as the audio which was actuallyscrambled, will be blanked. An on-screen display may appear on the TVstating that the viewer needs to subscribe to programming. Thisdesirably totally inhibits the casual viewer from both hearing andviewing the content.

[0045] In one embodiment of the present invention, the encrypted audiois transmitted as digitized packets over the A/V channel. Two (or more)audio streams are transmitted encrypted according to the two (or more)encryption systems in use by the system's set-top boxes. In order forthe two (or more) STBs to properly decrypt and decode their respectiveaudio streams, SI (system information) data are transmitted from thecable system's headend 122 that identifies the particular channel wherethe audio can be found using a transmitted Service Identifier to locatethe audio. This is accomplished by assigning the audio for system A is afirst packet identifier (PID) and assigning the audio for system B asecond packet identifier (PID). By way of example, and not limitation,the following program specific information (PSI) can be sent to identifythe location of the audio for two systems, one using NDS conditionalaccess and one using Motorola conditional access. Those skilled in theart will understand how to adapt this information to the otherembodiments of partial encryption described later herein.

[0046] The SI can be separately delivered to both legacy and non-legacyset-top boxes. It is possible to send SI information so that the legacyand non-legacy set-top boxes operate essentially without interference.In the SI delivered to legacy set-top boxes, the VCT (virtual channeltable) would state that the desired program, e.g. HBO referenced asprogram number 1, is on Service ID “1” and that the VCT access controlbit is set. The network information table (NIT) delivered to that firstSTB would indicate that Service ID “1” is at frequency=1234. In the SIdelivered to non-legacy set-top boxes, the VCT would state that thedesired program, e.g. HBO referenced as program number 1001, is onService ID “1001” and that the VCT access control bit is set. Thenetwork information table delivered to the non-legacy STB would indicatethat the Service ID “1001” is at frequency 1234. The following exemplaryprogram association Table PSI data are sent to both legacy andnon-legacy set-top boxes (in MPEG data structure format): PAT sent onPID=0x0000 PAT 0x0000 Transport Stream ID PAT version Program Number 1PMT 0x0010 Program Number 2 PMT 0x0020 Program Number 3 PMT 0x0030Program Number 4 PMT 0x0040 Program Number 5 PMT 0x0050 Program Number 6PMT 0x0060 Program Number 7 PMT 0x0070 Program Number 8 PMT 0x0080Program Number 9 PMT 0x0090 Program Number 1001 PMT 0x1010 ProgramNumber 1002 PMT 0x1020 Program Number 1003 PMT 0x1030 Program Number1004 PMT 0x1040 Program Number 1005 PMT 0x1050 Program Number 1006 PMT0x1060 Program Number 1007 PMT 0x1070 Program Number 1008 PMT 0x1080Program Number 1009 PMT 0x1090

[0047] The following exemplary program map table PSI data areselectively received by legacy and non-legacy set-top boxes (in MPEGdata structure format): PMT sent on PID=0x0010 PMT 0x0010 PMT Programnumber 1 PMT Section Version 10 PCR PID 0x0011 Elementary Stream StreamType (Video 0x02 or 0x80) Elementary PID (0x0011) Descriptor CADescriptor (ECM) for CA provider #1 Elementary Stream Stream Type (Audio0x81) Elementary PID (0x0012) Descriptor CA Descriptor (ECM) for CAprovider #1 PMT sent on PID=0x1010 PMT 0x1010 PMT Program number 1010PMT Section Version 10 PCR PID 0x0011 Elementary Stream Stream Type(Video 0x02 or 0x80) Elementary PID (0x0011) Descriptor CA Descriptor(ECM) for CA provider #2 Elementary Stream Stream Type (Audio 0x81)Elementary PID (0x0013) Descriptor CA Descriptor (ECM) for CA provider#2

[0048] Considering an example wherein it is desired to deliverprogramming in a system using either Motorola or Scientific Atlanta aswell as NDS CA, the above communications are consistent with the PSIdelivered by both Motorola and Scientific Atlanta in their CA systems,with only minor changes. The program association table (PAT) is changedto reference an additional program map table (PMT) for each program.Each program in this embodiment has two program numbers in the PAT. Inthe table above, program number 1 and program number 1001 are the sameprogram except that they will reference different audio PIDs and CAdescriptors. Changes in the system to create multiple PMTs and tomultiplex new PAT and PMT information with the data stream can be madeto appropriately modify the cable system headend equipment. Again, thoseskilled in the art will understand how to adapt these messages to otherpartial encryption schemes described herein. An advantage of thisapproach is that no special hardware or software is required for headendor for legacy and non-legacy set-top boxes to deliver audio that is bothlegacy and non-legacy encrypted using this scheme.

[0049] This technique deters the user from use of premium programmingwhich has not been paid for by rendering it inaudible, but a hacker mayattempt to tune the video. To combat this, the mechanisms employed inother encryption techniques consistent with the present invention (aswill be described later) can be employed simultaneously, if desired.Since closed captioning is generally transmitted as a part of the videodata, the user can still obtain readable audio information inconjunction with clear video. Thus, although adequate for someapplications, the present technique alone may not provide adequateprotection in all scenarios. In another embodiment, video packetscontaining closed captioning information as a part of the payload canadditionally be scrambled.

[0050] In an alternative embodiment, only the video may be dualencrypted with separate PIDs assigned to each set of encrypted video.While this may provide a more secure encryption for general programming(since video may be more important than audio), the amount of bandwidthsavings compared with full dual carriage is only approximately tenpercent, since only the audio is shared amongst all the set-top boxes.However, this approach might be used for certain content, e.g. adult andsports, and help reduce the bandwidth overhead for that content whilethe audio encryption approach may be used for other content types. Inthe Digital Satellite Service (DSS) transport standard used for theDirecTV™ service, the ardio packets can be identified for encryption byuse of the service channel identifier (SCID) which is consideredequivalent.

Time Slicing

[0051] Another embodiment consistent with the present invention isreferred to herein as time slicing and is illustrated in FIG. 3 assystem 200. In this embodiment, a portion of each program is encryptedon a time dependent basis in a manner that disrupts viewing of theprogram unless the user has paid for the programming. This embodiment ofthe invention can be implemented as partially encrypted video and clearaudio, clear video and partially encrypted audio or partially encryptedvideo and audio. The duration of the time slice that is encrypted, takenas a percentage of the total time, can be selected to meet any suitabledesired balance of bandwidth usage, security against hackers. Ingeneral, under any of the embodiments described herein, less than 100percent of the content is encrypted to produce a desired partialencryption. The following example details partially encrypted video andaudio.

[0052] By way of example, and not limitation, consider a system whichhas nine programs that are to be dual partially encrypted according tothe present exemplary embodiment. These nine channels are fed to thecable headend as a multiplexed stream of packets and are digitallyencoded using packet identifiers (PID) to identify packets associatedwith a particular one of the nine programs. In this example, assume thatthose nine programs have video PIDs numbered 101-109 and audio PIDsnumbered 201-209. The partial encryption, according to this embodimentis time multiplexed among the programs so that only packets from asingle program are encrypted at any given time. The method does not needto be content aware.

[0053] With reference to TABLE 1 below, an exemplary embodiment of atime slice dual encryption scheme consistent with an embodiment of theinvention is illustrated. For program 1 having primary video PID 101 andprimary audio PID 201, during the first time period, packets having PID101 and PID201 are encrypted using encryption system A, while the othersrepresenting the other programs are sent in the clear. In thisembodiment, secondary PIDs are also assigned to both the video and theaudio. The secondary PIDs are PID 111 for video and PID 211 for audiorespectively for program 1. The packets with the secondary PIDs areencrypted using encryption system B during the first time period. Thenext eight time periods are sent in the clear. Then for time period 10,packets having any of the above four PIDs are again encrypted followedby the next eight time periods being sent in the clear. In a similarmanner, during the second period of program 2 having primary video PID102 and primary audio PID 201 are encrypted using encryption system Aand packets with their associated secondary PIDs are encrypted usingencryption system B, and during the next eight time periods are sent inthe clear, and so on. This pattern can be seen clearly in TABLE 1 byexamination of the first nine rows. Both audio and video packets, oraudio alone or video alone can be encrypted according to this technique,without departing from the invention. Also, the audio and video can havetheir own individual encryption sequence. In TABLE 1, P1 indicates timeperiod number 1, P2 indicated time period number 2 and so on. EAindicates that the information is encrypted using CA system A and EBindicates that the information is encrypted using CA encryption systemB. TABLE 1 PROG. VIDEO PID AUDIO PID P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11P12 . . . 1 PID 101 PID 201 EA clear clear clear clear clear clear clearclear EA clear clear . . . 2 PID 102 PID 202 clear EA clear clear clearclear clear clear clear clear EA clear . . . 3 PID 103 PID 203 clearclear EA clear clear clear clear clear clear clear clear EA . . . 4 PID104 PID 204 clear clear clear EA clear clear clear clear clear clearclear clear . . . 5 PID 105 PID 205 clear clear clear clear EA clearclear clear clear clear clear clear . . . 6 PID 106 PID 206 clear clearclear clear clear EA clear clear clear clear clear clear . . . 7 PID 107PID 207 clear clear clear clear clear clear EA clear clear clear clearclear . . . 8 PID 108 PID 208 clear clear clear clear clear clear clearEA clear clear clear clear . . . 9 PID 109 PID 209 clear clear clearclear clear clear clear clear EA clear clear clear . . . 1 PID 111 PID211 EB EB . . . 2 PID 112 PID 212 EB EB . . . 3 PID 113 PID 213 EB EB .. . 4 PID 114 PID 214 EB . . . 5 PID 115 PID 215 EB . . . 6 PID 116 PID216 EB . . . 7 PID 117 PID 217 EB . . . 8 PID 118 PID 218 EB . . . 9 PID119 PID 219 EB . . .

[0054] In order to retain compatibility with an established legacyencryption system (encryption system A), the encrypted periods for eachof programs one through nine are encrypted using encryption system A.Legacy STB equipment will accept such partially encrypted A/V datastreams passing unencrypted packets and decrypting encrypted packetstransparently. However, it is desired to obtain dual encryption usingboth encryption system A and encryption system B. In order to achievethis, a specified program is assigned both primary PIDs (e.g., forprogram 1, video PID 101 and audio PID 201) and a secondary PID (e.g.,for program 1, video PID 111 and audio PID 211) to carry the elementarydata streams for a given premium channel.

[0055] With reference to FIG. 3, system 200 generally depicts thefunctionality of the cable system headend 222 wherein N channels ofclear video 204 at the headend 222 are provided to an intelligent switch216 (operating under control of a programmed processor) which routespackets that are to be transmitted in the clear to be assigned a primaryPID at 220. Packets that are to be encrypted are routed to bothconditional access system A encrypter 218 and to conditional accesssystem B encrypter 224. Once encrypted, these encrypted packets from 218and 224 are assigned primary or secondary PIDs respectively at 220.System information from 228 is multiplexed or combined with the clearpackets, the system A encrypted packets and the system B encryptedpackets and broadcast over the cable system 32.

[0056] For discussion purposes, if the period of the time slice is 100milli-seconds, then as shown in TABLE 1, there are on average one and afraction encrypted periods totaling 111 milli-seconds each second forall nine-programs. If the period is 50 milli-seconds, then there are onaverage two and a fraction encrypted periods totaling 111 milli-seconds.A non-subscribing box attempting to tune video would obtain a very poorimage if it could maintain any sort of image lock and the audio would begarbled.

[0057] The PSI for a partially scrambled stream is handled slightlydifferently from the dual audio encryption example above. Essentially,the same SI and PAT PSI information can be sent to both legacy andnon-legacy set-top boxes. The difference lies with the PMT PSIinformation. The legacy set-top box parses the PMT PSI and obtains theprimary video and audio PIDs as before. The non-legacy set-top boxobtains the primary PIDs like the legacy set-top box but must look atthe CA descriptors in the PMT PSI to see if the stream is partiallyscrambled. The secondary PID is scrambled specifically for a particularCA provider, consequently it makes sense to use the CA descriptorspecific to a particular CA provider to signal that PID. The inventioncan allow more than two CA providers to co-exist by allowing more thanone secondary PID. The secondary PID shall be unique to a particular CAprovider. The set-top box know the CA ID for the CA it has, and cancheck all CA descriptors for the relevant one for it.

[0058] While it is possible to send the secondary PID data as privatedata in the same CA descriptor used for the ECM, the preferredembodiment uses separate CA descriptors. The secondary PID is placed inthe CA PID field. This allows headend processing equipment to “see” thePID without having to parse the private data field of the CA descriptor.To tell the difference between the ECM and secondary PID CA descriptor,a dummy private data value can be sent. PMT sent on PID=0x0010 PMT0x0010 PMT Program number 1 PMT Section Version 10 PCR PID 0x0011Elementary Stream Stream Type (Video 0x02 or 0x80) Elementary PID(0x0011) Descriptor CA Descriptor (ECM) for CA provider #1 CA Descriptor(ECM) for CA provider #2 CA Descriptor (Secondary PID) for CA provider#2 Elementary Stream Stream Type (Audio 0x81) Elementary PID (0x0012)Descriptor CA Descriptor (ECM) for CA provider #1 CA Descriptor (ECM)for CA provider #2 CA Descriptor (Secondary PID) for CA provider #2

[0059] CA Descriptor for CA Provider #2 (ECM) Descriptor Tag:Conditional Access (0x09) Length: 4 Bytes Data CA System ID: 0x0942(2^(nd) CA provider) CA PID (0x0015) CA Descriptor for CA Provider #2(Secondary PID) Descriptor Tag: Conditional Access (0x09) Length: 5Bytes Data CA System ID: 0x1234 (2^(nd) CA provider) CA PID (0x0016)Private Data

[0060] Legacy STB 36 operating under CA system A receives the data,ignores the secondary PIDs, decrypts the packets encrypted under CAsystem A and presents the program to the television set 44. New ornon-legacy STB 236 receives the SI 228. It receives PSI 229 and uses thePMT to identify the primary and secondary PID, called out in the secondCA descriptor, associated with the program being viewed. The packetsencrypted under CA system A are discarded and the packets encryptedunder CA system B with the secondary PID are decrypted by CA system B240 and inserted into the clear data stream for decoding and display ontelevision set 244.

[0061]FIG. 4 illustrates one process for encoding at the cable systemheadend that can be used to implement an embodiment of the presentinvention wherein CA system A is the legacy system and CA system B isthe new system to be introduced. As a clear packet is received, at 250for a given program, if the packet (or frame) is not to be encrypted(i.e., it is not the current time slice for encryption for thisprogram), the clear packet (C) is passed on to be inserted into theoutput stream at 254. If the current packet is to be encrypted by virtueof the current packet being a part of the encryption time slice, thepacket is passed for encryption to both packet encryption process A 258and packet encryption process B 262. The encrypted packets fromencryption process A at 258 (EA) are passed on to 254 for insertion intothe output stream. The encrypted packets from encryption process B at262 (EB) are assigned a secondary PID at 264 for insertion into theoutput stream at 254. This is repeated for all packets in the program.

[0062]FIG. 5 illustrates a process used in the STB 236 having the newlyintroduced CA system B for decrypting and decoding the received datastream containing C, EA and EB packets having primary and secondary PIDsas described. When a packet is received at 272, it is inspected to seeif it has a the primary PID of interest. If not, the packet is examinedto see if it has the secondary PID of interest at 274. If the packet hasneither the primary or secondary PID, it is ignored or dropped at 278.Any intervening packets between the EA and EB packets that are not theprimary or secondary PID are discarded. It is an implementation andmainly a buffering issue whether a decoder can receive multiple EA or EBin a row before receiving the replacement matched EA or EB packet. Also,just as easy to detect for secondary packets that come before and notafter the primary packet. It is also possible to design a circuit whereeither case can happen—the secondary packet can before or after theprimary packet. If the packet has the primary PID of interest, thepacket is examined at 284 to determine if it is encrypted. If not, thepacket (C) is passed directly to the decoder at 288 for decoding. If thepacket is encrypted at 284, it is deemed to be an EA packet and isdropped or ignored at 278. In some implementations, the primary packet'sencryption does not get checked at 284. Rather, its simple positionrelative to the secondary packet can be checked at 284 to identify itfor replacement.

[0063] If the packet has the secondary PID at 274, the PID is remappedto the primary PID at 292 (or equivalently, the primary PID is remappedto the secondary PID value). The packet is then decrypted at 296 andsent to the packet decoder at 288 for decoding. Of course, those skilledin the art will recognize that many variations are possible withoutdeparting from the invention, for example, the order of 292 and 296 orthe order of 272 and 274 can be reversed. As mentioned earlier, 284 canbe replaced with a check of primary packet position with respect to thesecondary packet. Other variations will occur to those skilled in theart.

[0064] Legacy STB 36 operating under the encryption system A totallyignores the secondary PID packets. Packets with the primary PID aredecrypted, if necessary, and passed to the decoder without decryption ifthey are clear packets. Thus, a so called “legacy” STB operating underencryption system A will properly decrypt and decode the partiallyencrypted data stream associated with the primary PID and ignore thesecondary PID without modification. STBs operating under the encryptionsystem B are programmed to ignore all encrypted packets associated withthe primary PID and to use the encrypted packets transmitted with thesecondary PID associated with a particular channel.

[0065] Thus, each dual partially encrypted program has two sets of PIDsassociated therewith. If, as described, the encryption is carried out ona period-by-period basis, for the system shown with an appropriate timeslice interval, the picture will be essentially unviewable on a STB withneither decryption.

[0066] In order to implement this system in the headend 322 of FIG. 6,the SI and PSI can be modified for inclusion of a second set of CAdescriptor information. Legacy set-top boxes may not be able to tolerateunknown CA descriptors. Consequently, alternatively, in the set-top box,it may be possible to “hard code” offsets from the legacy CA PIDs forboth the content PIDs and/or the SI/PSI and ECM PIDs. Alternatively,parallel PSI may be sent. For example, an auxiliary PAT can be deliveredon PID 1000 instead of PID 0 for the non-legacy set-top boxes. It canreference auxiliary PMTs not found in the legacy PAT. The auxiliary PMTscan contain the non-legacy CA descriptors. Since auxiliary PMTs wouldnot be known to the legacy set-top boxes, there would not be anyinteroperation issue.

[0067] In systems where system A corresponds to legacy set-top boxesmanufactured by Motorola or Scientific Atlanta, no modifications to theSTBs are required. For the system B compliant STBs, for dual carriage ofpartially encrypted programs as described herein, the video and audiodecoder are adapted to listen to two PIDs each (a primary and asecondary PID) instead of just one. There may be one or more secondaryshadow PIDs, depending on the number of non-legacy CA systems in use,however a specific set-top box only listens to one of the secondary PIDsas appropriate for the CA method being used by that specific STB. Inaddition, ideally the encrypted packets from the PID carrying the mostlyclear video or audio are ignored. Since ignoring “bad packets” (thosethat cannot be readily decoded as is) may already be a function thatmany decoders perform, thus requiring no modification. For systems withdecoders that do not ignore bad packets, a filtering function can beused. It should be understood that the time slice encryption techniquecould be applied to just the video or the audio. Also, the video may betime slice encrypted while the audio is dual encrypted as in the earlierembodiment. The time slice technique may be applied to multiple programsconcurrently. The number of programs that encrypted during a period oftime is mainly an issue of bandwidth allocation, and although theexample discusses scrambling a single program at a time, the inventionis not limited by that. Other combinations of encryption techniquesdescribed in this document will also occur to those skilled in the art.

M^(TH) and N Packett Encryption

[0068] Another embodiment consistent with the present invention isreferred to herein as M^(th) & N packet encryption. This is a variationof the embodiment illustrated in FIG. 3 as system 200. In thisembodiment, packets of each PID representing a program are encrypted ina manner that disrupts viewing of the program unless the user has paidfor the programming. In this embodiment, M represents the number ofpackets between the start of an encryption event. N represents thenumber of packets that are encrypted in a row, once encryption takesplace. N is less than M. If M=9 and N=1, then every nine packets thereis an encryption event lasting 1 packet. If M=16 and N=2, then everysixteen packets there is an encryption event lasting two packets. Eachpacket to be dual partially encrypted is duplicated and processed usingCA system A 218 and CA system B 224 as in the previous embodiment. Thedifference in operation between this embodiment and the time slicingtechnique previously is in the operation of switch 216 to effect theselection of packets to encrypt under control of a programmed processor.

[0069] By way of example, and not limitation, consider a system whichhas nine channels of programming that are to be dual encrypted accordingto the present exemplary embodiment. These nine channels are digitallyencoded using packet identifiers (PID) to identify packets associatedwith a particular one of nine programs. In this example, assume thatthose nine programs have video PIDs numbered 101-109 and audio PIDsnumbered 201-209. The encryption, according to this embodiment is randomprogram-to-program so that packets from other programs may be encryptedat the same time. This is illustrated in TABLE 2 below in which M=6 andN=2 and in which only video is encrypted, but this should not beconsidered limiting. The method does not need to be content aware. InTABLE 2, PK1 indicated packet number 1, PK2 indicates packet number 2,and so on. TABLE 2 PROG. VIDEO PK1 PK2 PK3 PK4 PK5 PK6 PK7 PK8 PK9 PK10PK11 PK12 . . . 1 PID 101 EA EA clear clear clear clear EA EA clearclear clear clear . . . 2 PID 102 clear clear clear EA EA clear clearclear clear EA EA clear . . . 3 PID 103 clear clear EA EA clear clearclear clear EA EA clear clear . . . 4 PID 104 clear clear clear EA EAclear clear clear clear EA EA clear . . . 5 PID 105 clear clear EA EAclear clear clear clear EA EA clear clear . . . 6 PID 106 EA clear clearclear clear EA EA clear clear clear clear EA . . . 7 PID 107 EA EA clearclear clear clear EA EA clear clear clear clear . . . 8 PID 108 clear EAEA clear clear clear clear EA EA clear clear clear . . . 9 PID 109 EAclear clear clear clear EA EA clear clear clear clear EA . . . 1 PID 111EB EB EB EB . . . 2 PID 112 EB EB EB EB . . . 3 PID 113 EB EB EB EB . .. 4 PID 114 EB EB EB EB . . . 5 PID 115 EB EB EB EB . . . 6 PID 116 EBEB EB EB . . . 7 PID 117 EB EB EB EB . . . 8 PID 118 EB EB EB EB . . . 9PID 119 EB EB EB EB . . .

[0070] In the example of TABLE 2, each program is encrypted fullyindependently of the others using the M=6 and N=2 encryption scheme.Again, the illustrated example encrypts only the video, but audio couldalso be encrypted according to this or another arrangement. If appliedto just the video, audio may be dual scrambled or time slice encryptedas in earlier embodiments. Alternatively, if applied to just the audio,the video may be time sliced as in the earlier embodiment.

[0071] Those skilled in the art will recognize that many variations ofthe technique can be devised consistent with the partial scramblingconcepts disclosed herein. For example, a pattern of five clear followedby two encrypted followed by two clear followed by one encrypted(CCCCCEECCECCCCCEECCE . . . ) is consistent with variations of thepresent partial encryption concept, as are random, pseudo-random andsemi-random values for M and N may be used for selection of packets toencrypt. Random, pseudo-random or semi-random (herein collectivelyreferred to as “random” herein) selection of packets can make itdifficult for a hacker to algorithmically reconstruct packets in a postprocessing attempt to recover recorded scrambled content. Those skilledin the art will understand how to adapt this information to the otherembodiments of partial encryption described later herein. Some of theembodiments can be used in combination to more effectively secure thecontent.

Data Structure Encryption

[0072] Another partial encryption method consistent with embodiments ofthe present invention uses a data structure as a basis for encryption.By way of example and not limitation, one convenient data structure touse for encryption is an MPEG video frame. This is illustrated (againwith video only) in TABLE 3 below in which every tenth video frame isencrypted. In this embodiment, each program's ten frame encryption cycleis distinct from each other channel, but this should not be consideredlimiting. This concept can be viewed as a variation of the time slice orM^(th) and N partial encryption arrangement (or other pattern) basedupon video or audio frames (or some other data structure) with theexemplary embodiment having M=10 and N=1. Of course, other values of Mand N can be used in a similar embodiment. In TABLE 3, F1 representsframe number 1, F2 represents frame number 2 and so on. TABLE 3 PROG.VIDEO F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11 F12 . . . 1 PID 101 EA clearclear clear clear clear clear clear clear clear EA clear . . . 2 PID 102clear clear clear EA clear clear clear clear clear clear clear clear . .. 3 PID 103 clear clear EA clear clear clear clear clear clear clearclear clear . . . 4 PID 104 clear clear clear clear EA clear clear clearclear clear clear clear . . . 5 PID 105 clear clear clear EA clear clearclear clear clear clear clear clear . . . 6 PID 106 EA clear clear clearclear clear clear clear clear clear EA clear . . . 7 PID 107 clear EAclear clear clear clear clear clear clear clear clear EA . . . 8 PID 108clear EA clear clear clear clear clear clear clear clear clear EA . . .9 PID 109 EA clear clear clear clear clear clear clear clear clear EAclear . . . 1 PID 111 EB EB . . . 2 PID 112 EB . . . 3 PID 113 EB . . .4 PID 114 EB . . . 5 PID 115 EB . . . 6 PID 116 EB EB . . . 7 PID 117 EBEB . . . 8 PID 118 EB EB . . . 9 PID 119 EB EB . . .

[0073] Thus, again each encrypted program has two sets of PIDsassociated therewith. If, as described, the encryption is carried out ona period-by-period basis, for the system shown, the picture will beessentially unviewable. For a nine program system at 30 frames persecond as depicted, approximately three frames per second will beencrypted. For viewers who are not entitled to view the program, theirSTB will be unable to capture much more than an occasional frozen frameas the STB constantly attempts to synchronize and recover. Viewers whohave subscribed to the programming will be able to readily view theprogramming. The bandwidth cost for such an encryption arrangementdepends upon the frequency with which the encryption is applied. In theabove example, an extra factor of {fraction (1/9)} of data aretransmitted for each program. In this example, approximately oneprogram's worth of bandwidth is used. With a greater number of programs,fewer packets per program are encrypted and the security of theencryption system may degrade somewhat. As in the randomized M and Nmethod, random frames may be selected. Choosing random frames, in thevideo case, would help guarantee that all frame types would beaffected—intra-coded frames (I frames), predictive-coded (P frames),Bi-directional-coded (B frames) and DC frames.

[0074] In a variation of the invention, it may be possible to encryptfewer packets to achieve an acceptable level of security. That is,perhaps in a system of nine programs, only one frame per second may needto be encrypted to achieve acceptable levels of security. In such asystem, the overhead becomes one encrypted period per second per programor approximately {fraction (1/30)} of data transmitted in overhead. Thislevel of overhead is a dramatic improvement over the 50% loss ofbandwidth associated with full dual carriage of encryption under twoencryption systems. In another variation of the invention, it may bepossible to encrypt only certain video frames to achieve an acceptablelevel of security. For example, for MPEG content, only intra-codedframes (I frames) may be scrambled to further reduce the bandwidthoverhead and still maintain an acceptable level of security. These offersignificant improvement over the bandwidth required for full dualcarriage.

Critical Packet Encryption

[0075] Substantial efficiency in bandwidth utilization can be achievedby use of a selective packet-by-packet dual encryption technique. Inthis technique, packets are selected for encryption based upon theirimportance to the proper decoding of the audio and/or video of theprogram content.

[0076] This embodiment can reduce the bandwidth requirement comparedwith full dual carriage of encrypted content by only scrambling a smallfraction of the packets. Clear packets are shared between the two (ormore) dual carriage PIDs. In one preferred embodiment, as will bedisclosed, less that about one percent of the total content bandwidth isused. In a system with a legacy encryption scheme, clear program contentpackets can be received by both legacy and new set-top boxes. Asmentioned before, encrypted packets are dual carried and processed bythe respective set-top boxes with the appropriate CA. Each CA system isorthogonal. Key sharing is not required and different key epochs may beused by each CA system. For example, a system with Motorola'sproprietary encryption can generate fast changing encryption keys usingthe embedded security ASIC, while an NDS smart card based system cangenerate slightly slower changing keys. This embodiment works equallywell for Scientific Atlanta and Motorola legacy encryption.

[0077] Referring now to FIG. 6, a block diagram of a system consistentwith an embodiment of the present invention in which portions ofprogramming are dual encrypted on a packet-by-packet basis isillustrated as system 300. In this system, packets of each program aredual encrypted using, for example, legacy CA system A and CA system B.The packets that are encrypted are selected based upon their importanceto the proper decoding of the video and/or audio stream.

[0078] In the system illustrated in FIG. 6, the cable system headend 322selects A/V content 304 packets at a packet selector 316 for encryption.Packets selected for encryption are chosen so that their non-receipt (bya non-paying decoder) would severely affect the real-time decoding of aprogram, and any possible post processing of recorded content. That is,only critical packets are encrypted. For the video and audio, this canbe accomplished by encrypting “start of frame” transport stream packetscontaining PES (packetized elementary stream) headers and other headersas part of the payload, since without this information, the STB decodercannot decompress the MPEG compressed data. MPEG2 streams identify“start of frame” packets with the “Packet Unit Start Indicator” in thetransport header. Generally, packets carrying a payload that contains agroup of pictures header or a video sequence header can be used toeffect the present scrambling technique.

[0079] MPEG (Moving Pictures Expert Group) compliant compressed videorepackages the elementary data stream into the transport stream insomewhat arbitrary payloads of 188 bytes of data. As such, the transportstream packets containing a PES header can be selected for encryption atselector 316 and dual encrypted by both the CA system A encrypter 318and the CA system B encrypter 324. Packets to be dual partiallyencrypted are duplicated and the PIDs of duplicate packets encrypted byencrypter 324 are remapped at 330 to a secondary PID as in the previousembodiment. The remaining packets are passed in the clear. The clearpackets, system A encrypted packets, system B encrypted packets andsystem information 328 are multiplexed together for broadcast over thecable system 32.

[0080] As with the previous system, the legacy STB 36 receives cleardata and data encrypted under CA encryption system A and transparentlypasses unencrypted data combined with data decrypted by CA decryption A40 to its decoder. In the new STB 336, the program is assigned to both aprimary and a secondary PID. The clear packets with the primary PID arereceived and passed to the decoder. The encrypted packets with theprimary PID are discarded. Encrypted packets with the secondary PID aredecrypted and then recombined with the data stream (e.g., by remappingthe packets to the primary PID) for decoding.

[0081] Using video is used as an example, each sample is known as aframe and the sample rate is typically 30 frames per second. If thesamples are encoded to fit into 3.8 Mbps, each frame would occupy 127Kbits of bandwidth. This data is sliced for MPEG transport into packetsof 188 bytes with the first packet(s) of each frame containing theheader used for instructions to process the body of the frame data. Dualencrypting just the first header packet (1504 additional bits) requiresonly 1.2% (1504/127K) of additional bandwidth. For high definition (19Mbps) streams the percentage is even less.

[0082] As previously stated, transport stream packets containing a PESheader are the preferred target for encryption according to the presentembodiment. These packets contain sequence headers, sequence extensionheaders, picture headers, quantization and other decode tables that alsofall within the same packet. If these packets cannot be decoded (i.e.,by a hacker attempting to view unauthorized programming without payingthe subscription charges), not even small portions of the program can beviewed. In general, any attempt to tune to the program will likely bemet with a blank screen and no audio whatsoever since known decoderintegrated circuits use the PES header to sync up to an elementarystream such as video and audio in real-time. By encrypting the PESheader, the decoding engine in an un-authorized set-top box cannot evenget started. Post processing attacks, e.g. on stored content, arethwarted by critical dynamically changing information in the packetcontaining the PES header. Those skilled in the art will appreciate thatfor implementation of this embodiment of the invention, other criticalor important packets or content elements may also be identified forencryption that could severely inhibit unauthorized viewing withoutdeparting from the present invention. For example, MPEG intra-coded or Iframe picture packets could be encrypted to inhibit viewing of the videoportion of the program. Embodiments the present invention may be used inany combination with other embodiments, e.g. scrambling the packetcontaining the PES header as well as random, M^(th) and N, or datastructure encryption of the other packets. Critical packet encryptionmay be applied to video encryption, while a different method may beapplied to audio. Audio could be dual encrypted, for instance. Othervariations within the scope of the present invention will occur to thoseskilled in the art.

[0083]FIG. 7 is a flow chart depicting an exemplary encoding processsuch as that which would be used at headend 322 of FIG. 6. When atransport stream packet is received at 350, the packet is examined todetermine if it meets a selection criteria for encryption. In thepreferred embodiment, this selection criteria is the presence of a PESheader as a portion of the packet payload. If not, the packet is passedas a clear unencrypted packet (C) for insertion into the output datastream at 354. If the packet meets the criteria, it is encrypted underCA encryption system A at 358 to produce an encrypted packet EA. Thepacket is also duplicated and encrypted under CA encryption system B at362 to produce an encrypted packet. This encrypted packet is mapped to asecondary PID at 366 to produce an encrypted packet EB. Encryptedpackets EA and EB are inserted into the output data stream along withclear packets C at 354. Preferably, the EA and EB packets are insertedat the location in the data stream where the single original packet wasobtained for encryption so that the sequencing of the data remainsessentially the same.

[0084] When the output data stream from 354 is received at an STBcompliant with CA encryption system B such as 336 of FIG. 6, a processsuch as that of FIG. 8 (which is similar to that of FIG. 5) can beutilized to decrypt and decode the program. When a packet is receivedhaving either the primary or the secondary PID at 370, a determinationis made as to whether the packet is clear (C) or encrypted under systemA (EA) at 370 or encrypted under system B (EB) at 374. If the packet isclear, it is passed directly to the decoder 378. In some embodiments,the relative position of the primary packet, before or after, to thesecondary packet may be used to signal a primary packet for replacementin the stream. A check of the scrambling state of the primary packet isnot specifically required. If the packet is an EA packet, it is droppedat 380. If the packet is an EB packet, it is decrypted at 384. At thispoint, the secondary PID packets and/or the primary PID packets areremapped to the same PID at 388. The decrypted and clear packets aredecoded at 378.

[0085] The dual partial encryption arrangement described above cangreatly reduce the bandwidth requirements over that required for fulldual carriage. Encrypting the PES header information can be effective insecuring video and audio content, while allowing two or more CA systemsto independently “co-exist” on the same cable system. Legacy system Aset-top boxes are unaffected, and system B set-top boxes require only anminor hardware, firmware, or software enhancement to listen for two PIDseach for video and audio. Each type of STB, legacy and non-legacy,retains its intrinsic CA methodology. Headend modification is limited toselecting content for encryption, introducing the second encrypter, andproviding a means to mix the combination into a composite output stream.

[0086] In one embodiment, the headend equipment is configured toopportunistically scramble as much of the content as the bandwidth willallow, and not just the critical PES headers. These additional scrambledpackets would be either in the PES payload or other packets throughoutthe video/audio frame to provide even further security of the content.

SI Encryption

[0087] Turning now to FIG. 9, one embodiment of a system that minimizesthe need for any additional bandwidth is illustrated as system 400. Inthis embodiment, the system takes advantage of the fact that systeminformation (SI) 428 is required for a set-top box to tune programming.In a cable system, SI is sent in the out-of-band, a frequency set asidefrom the normal viewing channels. It is possible to also sent itin-band. If sent in-band, the SI 428 is replicated and sent with eachstream. For discussion purposes, assume that the SI delivered to“legacy” set-top boxes from previous manufacturers is separate from theSI delivered to set-tops from new manufacturers such as STB 436.Consequently, each version of the SI can be independently scrambled asillustrated using conditional access system A 418 and conditional accesssystem B 424. The clear video 404 and clear audio 406 are delivered inthe clear, but in order to understand how to find them, the SIinformation 428 is needed.

[0088] The SI delivers information about channel names and program guideinformation such as program names and start times, etc. . . . as well asthe frequency tuning information for each channel. Digital channels aremultiplexed together and delivered at particular frequencies. In theembodiment of the invention, the SI information is encrypted, and onlymade available to authorized set-top boxes. If the SI information is notreceived to allow knowledge of the location of all the A/V frequenciesin the plant, then tuning cannot take place.

[0089] To frustrate a hacker who might program a set-top box to trial orscan frequencies, the frequencies for the channels can be offset fromthe standard frequencies. Also, the frequencies can be dynamicallychanged on a daily, weekly or other periodic or random basis. A typicalcable headend may have roughly 30 frequencies in use. Each frequency istypically chosen to avoid interference between, among other things, eachother, terrestrial broadcast signals, and frequencies used by clocks ofthe receiving equipment. Each channel has at least 1 independentalternate frequency that if used would not could not cause interference,or cause the frequency of adjoining channels to be changed. The actualpossible frequency maps are therefore 2³⁰ or 1.07×10⁹. However, a hackermight simply quickly try both frequencies on each tune attempt for eachof the 30 channels or so. If successful in locating a frequency withcontent, the hacker's set-top box can then parse the PSI 429 to learnabout the individual PIDs that make up a program. The hacker will havedifficulty learning that “program 1” is “CNN”, and that “program 5” is“TNN”, and so on. That information is sent with the SI, which as statedabove is scrambled and otherwise unavailable to the un-authorizedset-top box. However, a persistent hacker might yet figure those out byselecting each one and examining the content delivered. So in order tofrustrate the identification of channels, the assignment of a programwithin a single stream can move around, e.g. program 2 and program 5swapped in the example above so that “program 1” is “TNN” and “program5” is “CNN”. Also, it is possible to move programs to entirely differentstreams with entirely new program groupings. A typical digital cableheadend can deliver 250 programs of content including music. Each can beuniquely tuned. The possible combinations for re-ordering are 250!(factorial). Without a map of the content provided by either thedelivered SI or by a hacker, the user is faced with randomly selectingeach program in a stream to see if it is the one interest.

[0090] Thus, at headend 422, the video signal 404 and the audio signal406 are provided in the clear (unencrypted) while the SI 428 is providedto multiple CA systems for delivery over the cable network. Thus, in theexemplary system 400, clear SI 428 is provided to an encryption system428 that encrypts SI data using encryption system A. Simultaneously,clear SI 428 is provided to encryption system 424 that encrypts the SIdata using encryption system B. Clear video and audio are thenmultiplexed along with encrypted SI from 418 (SI A) and encrypted audiofrom 424 (SI B) out of band system information 428.

[0091] After distribution through the cable system 32, the video, theaudio, system information A and system information B are all deliveredto set-top boxes 36 and 436. At STB 36, the encrypted SI is decrypted atCA system A 40 to provide tuning information to the set-top box. Theset-top box tunes a particular program to allow it to be displayed ontelevision set 44. Similarly, at STB 436, the encrypted SI is decryptedat CA system B 440 to provide tuning information for the set-top box,allow a particular program to be tuned and displayed on television set444.

[0092] An advantage of this approach is that no additional A/V bandwidthis required in the content delivery system, e.g. cable system. Only theSI is dual carried. No special hardware is required. Any offsetfrequencies from the standard ones can be easily accommodated by mosttuners. SI decryption can be performed in software or can be aided byhardware. For example, legacy Motorola set-top boxes have an ability todescramble the SI delivered in the Motorola out-of-band using a hardwaredecrypter built into the decoder IC chip.

[0093] A determined hacker can potentially use a spectrum analyzer onthe coax cable to learn where the A/V channels are located. Also, it maybe possible for the hacker to program a set-top box to auto-scan thefrequency band to learn where the A/V channels are—a relatively slowprocess. If the A/V channel frequencies changed dynamically, then thatcould foil the hackers, since they would need to be constantly analyzingor scanning the band. Also, the program numbers and assigned PIDs canvary. However, dynamically changing frequencies, program numbers, andPIDs might create operational difficulties to a service provider, e.g.cable operator.

Generalized Representation

[0094] Each of the above techniques can be represented generically bythe system 500 of FIG. 10. This system 500 has a cable system headend522 with clear video 504, clear audio 506, SI 528, and PSI 529 any ofwhich can be selectively switched through an intelligent processorcontrolled switch 518, which also serves to assign PIDs (in embodimentsrequiring PID assignment or reassignment), to conditional access systemA 504 or conditional access system B 524 or passed in the clear to thecable system 32. As previously, the program or SI encrypted according tothe legacy CA system A can be properly decoded by STB 36. The CA systemB encrypted information is understood by STBs 536 and decrypted anddecoded accordingly, as described previously.

PID Mapping Considerations

[0095] The PID mapping concepts described above can be generally appliedto the dual partial encryption techniques described herein, whereneeded. At the cable headend, the general concept is that a data streamof packets is manipulated to duplicate packets selected for encryption.Those packets are duplicated and encrypted under two distinct encryptionmethods. The duplicated packets are assigned separate PIDs (one of whichmatches the legacy CA PID used for clear content) and reinserted in thelocation of the original selected packet in the data stream fortransmission over the cable system. At the output of the cable systemheadend, a stream of packets appears with the legacy encrypted packetsand clear packets having the same PID. A secondary PID identifies thepackets that are encrypted under the new encryption system. In additionto the PID remapping that takes place at the headend, MPEG packetsutilize a continuity counter to maintain the appropriate sequence of thepackets. In order to assure proper decoding, this continuity countershould be properly maintained during creation of the packetized datastream at the headend. This is accomplished by assuring that packetswith each PID are assigned continuity counters sequentially in a normalmanner. Thus, packets with the secondary PID will carry a separatecontinuity counter from those of the primary PID. This is illustratedbelow in simplified form where PID 025 is the primary PID and PID 125 isthe secondary PID, E represents an encrypted packet, C represents aclear packet, and the end number represents a continuity counter. 025C04025E05 125E11 025C06 025C07 025C08 025C09 125- E12

[0096] In this exemplary segment of packets, packets with PID 025 areseen to have their own sequence of continuity counters (04, 05, 06, 07,08, 09, . . . ). Similarly, the packets with secondary PID 125 also havetheir own sequence of continuity counters (11, 12, . . . ).

[0097] At the STB, the PIDs can be manipulated in any number of ways tocorrectly associate the encrypted packets with secondary PID with thecorrect program. In one implementation, the packet headers of an inputstream segment illustrated below: 025C04 025E05 125E11 025C06 025C07025C08 025C09 025- E10

[0098] are manipulated to create the following output stream segment:125C04 025E11 125E05 125C06 125C07 125C08 125C09 125- E10

[0099] The primary PIDs (025) in the input stream are replaced with thesecondary PID (125) for the clear packets (C). For the encryptedpackets, the primary PID and secondary PID are retained, but thecontinuity counters are swapped. Thus, the stream of packets can now beproperly decrypted and decoded without errors caused by loss ofcontinuity using the secondary PID. Other methods for manipulation ofthe PIDs, e.g. mapping the PID (125) on the scrambled legacy packet to aNOP PID (all ones) or other PID value not decoded, and the continuitycounters can also be used in embodiments consistent with the presentinvention.

[0100] The primary and secondary PIDs are conveyed to the STBs in theprogram map table (PMT) transmitted as a part of the program systeminformation (PSI) data stream. The existence of a secondary PID can beestablished to be ignored by the STB operating under CA encryptionsystem A (the “legacy” system), but new STBs operating under CAencryption system B are programmed to recognize that secondary PIDs areused to convey the encrypted part of the program associated with theprimary PID. The set-top boxes are alerted to the fact that thisencryption scheme is being used by the presence of a CA descriptor inthe elementary PID “for loop” of the PMT. There typically would be a CAdescriptor for the video elementary PID “for loop”, and another one inthe audio elementary PID “for loop”. The CA descriptor uses a PrivateData Byte to identify the CA_PID as either the ECM PID or the secondaryPID used for partial scrambling, thus setting up the STB operating undersystem B to look for both primary and secondary PIDs associated with asingle program. Since the PID field in the transport header is thirteenbits in length, there are 2¹³ or 8,192 PIDs available for use, any sparePIDs can be utilized for the secondary PIDs as required.

[0101] In addition to the assignment of a PID for each program componentor selected portion thereof, a new PID may be assigned to tag ECM dataused in the second encryption technique. Each PID number assigned can benoted as a user defined stream type to prevent disrupting operation of alegacy STB. MPEG defines a reserved block of such numbers for userdefined data stream types.

[0102] While conceptually the PID mapping at the cable headend is asimple operation, in practice the cable headend equipment is oftenalready established and is therefore modified to accomplish this task ina manner that is minimally disruptive to the established cable systemwhile being cost effective. Thus, the details of the actualimplementation within the cable system headend are somewhat dependentupon the actual legacy hardware present in the headend, examples ofwhich are described in greater detail below.

Headend Implementations

[0103] Those skilled in the art will appreciate that the abovedescriptions as related to FIGS. 2, 3, 6, 9 and 10 are somewhatconceptual in nature and are used to explain the overall ideas andconcepts associated with the various embodiments of the presentinvention. In realizing a real world implementation of the presentinvention, those skilled in the art will recognize that a significantreal world issue to contend with is providing a cost effectiveimplementation of the various partial encryption methods within existinglegacy headend equipment at established cable providers. Taking two ofthe primary legacy cable systems as examples, the following describeshow the above techniques can be implemented at a cable headend.

[0104] First, consider a cable system headend using a Motorola brandconditional access system. In such a system the modifications shown inFIG. 11 can be done to provide a cost effective mechanism for partialdual encryption implementation. In a typical Motorola system, a HITS(Headend In The Sky) or similar data feed is provided from a satellite.This feed provides aggregated digitized content that is supplied tocable providers and is received by a receiver/descrambler/scramblersystem 604 such as the Motorola Integrated Receiver Transcoder (IRT)models IRT 1000 and IRT 2000, and Motorola Modular Processing System(MPS). A clear stream of digitized television data can be obtained fromthe satellite descrambler functional block 606 of thereceiver/descrambler/scrambler 604. This clear stream can be manipulatedby a new functional block shown as packet selector/duplicator 610. Thisnew block 610 may be implemented as a programmed processor or may beotherwise implemented in hardware, software or a combination thereof.

[0105] Packet selector/duplicator 610 selects packets that are to bedual encrypted under any of the above partial dual encryption methods.Those packets are then duplicated with new PIDs so that they can belater identified for encryption. For example, if packets at the input of610 associated with a particular program have PID A, then packetselector/duplicator 610 identifies packets to be encrypted andduplicates those packets and remaps them to PIDs B and C respectively,so that they can be identified later for encryption under two differentsystems. Preferably, the duplicate packets are inserted into the datastream adjacent one another in the location of the originally duplicatedpacket nowwith PID C so that they remain in the same order originallypresented (except that there are two packets where one previouslyresided in the data stream). Assume, for the moment, that the new CAsystem to be added is NDS encryption. In this case, PID A will representclear packets, PID B will represent NDS encrypted packets and PID C willrepresent Motorola encrypted packets. The packets having PID B may beencrypted under the NDS encryption at this point in 610 or may beencrypted later.

[0106] The packets with PIDs B and C are then returned to the system 604where packets with PID C are encrypted under Motorola encryption atcable scrambler 612 as instructed by the control system 614 associatedwith the Motorola equipment. The output stream from cable scrambler 612then proceeds to another new device—PID remapper and scrambler 620,which receives the output stream from 612 and now remaps the remainingpackets with PID A to PID C and encrypts the PID B packets under the NDSencryption algorithm under control of control system 624. The outputstream at 626 has clear unencrypted packets with PID C and selectedpackets which have been duplicated and encrypted under the Motorolaencryption system with PID C along with encrypted packets under the NDSencryption system with PID B. This stream is then modulated (e.g.,Quadrature Amplitude Modulated and RF modulated) for distribution overthe cable system. The preferred embodiment maps the unencrypted packetson PID A to match the scrambled packets on PID C because the audio andvideo PIDs called out in legacy program specific information (PSI) iscorrect that way. The control computer, the scrambler, and legacyset-top boxes only know about PID C. Alternatively, the scrambledpackets on PID C could be mapped back to PID A, but this would likelymean editing the PSI, that was automatically generated, to map the PIDnumbers from PID C back to PID A in the PID remapper and scrambler 620.

[0107] In the above example, the PID remapper and scrambler 620 may alsobe used to demultiplex PSI information, modify it to reflect theaddition of the NDS encryption (through the use of CA descriptors in thePMT) and multiplex the modified PSI information back into the datastream. The ECMs to support NDS encryption may also be inserted into thedata stream at PID remapper and scrambler 620 (or could be inserted bypacket selector/duplicator 610).

[0108] Thus, in order to add NDS encryption (or another encryptionsystem) to a cable system headend using Motorola equipment, packets areduplicated and PIDs are remapped in the data stream from the satellitedescrambler. The remapped PIDs are then used to identify packets thatare to be scrambled under each CA system. Once the legacy systemencryption has taken place, the clear PID is then remapped so that bothclear and encrypted packets in the legacy system share the same PID (orPIDs). PID remapping as in 620 and packet selection and duplication asin 610 can be implemented using a programmed processor or using customor semi-custom integrated circuitry such as an application specificintegrated circuit or a programmable logic device or field programmablegate array. Other implementations are also possible without departingfrom the present invention.

[0109]FIG. 12 depicts a similar equipment configuration such as thatused in implementing the partial dual encryption of the presentinvention in a Scientific Atlanta based cable headend. In thisembodiment, the HITS feed or similar is received at IRD 704 whichincorporates a satellite descrambler 706. This may be a Motorola IRT orMPS with only the satellite descrambler function enabled. The output ofthe satellite descrambler 706 again provides a clear data stream thatcan be manipulated by a new packet selector/duplicator 710 which selectspackets to be encrypted, duplicates them and maps the PIDs of theduplicate packets to new PIDs. Again, for example, packets to remain inthe clear are assigned PID A, packets to be encrypted under the newsystem (e.g., NDS) are assigned PID B and packets to be encrypted underthe Scientific Atlanta encryption system are assigned PID C. The packetswith PID B may be encrypted at this point under the NDS encryptionsystem.

[0110] The stream of packets is then sent to a multiplexer 712 (e.g., aScientific Atlanta multiplexer) where the packets having PID C areencrypted under the Scientific Atlanta encryption system at 714 undercontrol of control system 718 associated with multiplexer 712. Thestream of data is then supplied internal to multiplexer 712 to a QAMmodulator 720. In order to properly remap the packets, the QAM modulatedsignal at the output of multiplexer 712 is provided to a new processorsystem 724 where the QAM modulated signal is demodulated at a QAMdemodulator 730 and the clear PID A packets are remapped to PID C at PIDremapper 734 under control of a control system 738. Encryption under theNDS encryption algorithm can also be carried out here rather than in710. The data stream with remapped PIDs and dual partial encryption isthen QAM and RF modulated at 742 for distribution over the cable system.

[0111] In the above example, the PID remapper and scrambler 734 may alsobe used to demultiplex PSI information, modify it to reflect theaddition of the NDS encryption (adding the CA descriptors to the PMT)and multiplex the modified PSI information back into the data stream.The ECMs to support NDS encryption may also be inserted into the datastream at PID remapper and scrambler 734 (or could be inserted by packetselector/duplicator 710). PID remapping and or scrambling as in 734along with QAM demodulation and QAM modulation as in 730 and 742respectively, and packet selection and duplication as in 710 can beimplemented using a programmed processor or using custom or semi-customintegrated circuitry such as an application specific integrated circuitor a programmable logic device or field programmable gate array. Otherimplementations are also possible without departing from the presentinvention.

[0112] The above embodiments of the present invention allow legacyscrambling equipment to scramble only the packets desired in anelementary stream instead of the entire elementary stream. Thescrambling of certain packets of an elementary stream is accomplished byusing a PID number for packets that are not going to be scrambled, e.g.,PID A. Packets that will be scrambled will be placed on PID C. Thescrambling equipment will scramble the packets on PID C (the ones thathave been selected for scrambling). After the scrambling has takenplace, the unscrambled packets have the PID number mapped to the same asthe scrambled packet—PID A becomes PID C. The legacy set-top boxes willreceive an elementary stream with both scrambled and un-scrambledpackets.

[0113] The packets in these embodiments are handled as a stream. Theentire stream is sent to the legacy scrambling equipment for scrambling.This keeps all of the packets in exact time synchronous order. Ifpackets were extracted from a stream and sent to the legacy scramblingequipment, time jitter might be introduced. The present embodimentavoids that problem by keeping all the packets in a stream. Theembodiment does not require cooperation from the legacy scramblingequipment provider because that equipment is not involved in theremapping of packets—from PID A to PID C. This remapping is preferablebecause the PID called out by the PSI generated by the legacy scramblingsystem does not need to change. The legacy system knows about PID C, butnot PID A. The entire elementary stream to be scrambled by the legacyscrambling equipment is found on a single PID that the scrambling systemhas been instructed to scramble.

[0114] In the above examples, the use of NDS as the second encryptionsystem should not be considered limiting. Moreover, although two widelyused systems—Motorola and Scientific Atlanta have been depicted by wayof example, similar modifications to legacy systems to permit PIDremapping and dual partial encryption can be used. In general, thetechnique described above involves the process generally described as800 in FIG. 13. A feed is received at 806 which is descrambled as it isreceived at 810 to produce a clear data stream of packets. At 814,packets are selected according to the desired partial dual encryptiontechnique (e.g., audio only, packets containing PES header, etc.). At818, the selected packets are duplicated and the duplicate pairs areremapped to two new PIDs (e.g., PID B and PID C). The duplicated packetsare then encrypted based upon PID (that is, PID C is encrypted accordingto legacy encryption and PID B is encrypted according to the newencryption system) at 822. The clear packets (e.g., PID A) are thenremapped to the same PID as the legacy encrypted PID (PID C) at 826.

[0115] The order in which some of the elements of the process of FIG. 13are carried out can vary according to the particular legacy system beingmodified to accommodate the particular dual encryption arrangement beingused. For example, encryption under a new encryption system can becarried out either at the time of duplication or later at the time ofremapping the legacy packets, as illustrated in FIGS. 11 and 12.Additionally, various demodulation and re-modulation operations can becarried out as needed to accommodate the particular legacy system athand (not shown in FIG. 13).

Set-top Box Implementations

[0116] Several set-top box implementations are possible within the scopeof the present invention. The method used at the headend to selectpackets for encryption is irrelevant to the STB.

[0117] One such implementation is illustrated in FIG. 14. In thisembodiment, packets from a tuner and demodulator 904 are provided to adecoder circuit 908's demultiplexer 910. The packets are buffered into amemory 912 (e.g., using a unified memory architecture) and processed bythe STB's main CPU 916 using software stored in ROM memory 920.

[0118] Selected PIDs can be stripped from the incoming transport via theSTB's PID filter, decrypted and buffered in SDRAM, similar to theinitial processing required in preparation for transfer to an HDD in aPVR application. The host CPU 916 can then “manually” filter thebuffered data in SDRAM for elimination of the packets containingunneeded PIDs. There are some obvious side effects to this process.

[0119] The host overhead is estimated to be about 1% of the bandwidth ofthe CPU. In the worst case, this is equivalent to 40K bytes/Second for a15 Mbit/S video stream. This reduction is possible since at most only 4bytes of each packet is evaluated and the location is on 188 byteintervals so the intervening data does not have to be considered. Eachpacket header in SDRAM can therefore be directly accessed through simplememory pointer manipulation. Additionally, Packets are cached in blocksand evaluated en masse to reduce task switching of the host. This wouldeliminate an interrupt to other tasks upon the reception of each newpacket. This may produce a increased latency for starting decode of astream upon channel change to allow time for cache fill. This may benegligible depending upon the allocated SDRAM cache buffer size.

[0120] The host filtered packets in the SDRAM buffer are thentransferred to the A/V Queue through existing hardware DMA processes andmimics a PVR implementation. The filtered packets are then provided tothe decoder 922 for decoding.

[0121] A second technique for implementation in a set-top box isillustrated in FIG. 15. Since RISC processor A/V decoder module in 930processes the partial transport PIDs and strips/concatenates for decode,the firmware within decoder IC 930 can be altered to exclude individualpackets in a partial transport stream based upon criteria in each packetheader. Alternatively, the demultiplexer 910 can be designed to excludethe packets. Legacy scrambled packet(s) pass through the CA module stillencrypted. By using the decoder IC 930 to perform the removal of thelegacy scrambled packets and assuming that the packets encrypted underthe new encryption algorithm (e.g., NDS) is immediately adjacent thelegacy encrypted packet (or at least prior to next primary stream videopacket) then the pruning of the legacy packet in effect accomplishes themerging of a single, clear stream into the header strip and video queue.

[0122] A third technique for implementation of partial decryption in aset-top box is illustrated in FIG. 16. In this embodiment, the PIDremapping is carried out either within a circuit such as an ASIC, FieldProgrammable Gate Array (FPGA), or a programmable logic device (PLD) 938or other custom designed circuit placed between the tuner anddemodulator 904 and the decoder IC 908. In a variation of thisembodiment, the decoder IC 908 can be modified to implement the PIDremapping within demultiplexer 940. In either case, the legacy encryptedpackets are dropped and the non-legacy packets re-mapped either incircuit 938 or demultiplexer 940.

[0123] This third technique can be implemented in one embodiment usingthe PLD depicted in FIG. 17. This implementation assumes that there willbe not be more than one encrypted packet of a particular PID appearingin a row, thus, the implementation could be modified to accommodatebursts of encrypted packets such as with the M and N^(th) encryptionarrangement described above (as will be explained later). The inputstream passes through a PID identifier 950 which serves to demultiplexthe input stream based upon PID. Primary PID packets are checked forcontinuity at 958. If a continuity error is detected, the error is notedand the counter is reset at 960.

[0124] The original input packet stream contains packets tagged withmany PIDs. The PID identifier 950 separates packets with the two PIDs ofinterest (primary and secondary PIDs) from all other packets. Thiscapability can be scaled to process multiple PID pairs. These otherpackets are bypassed directly to the revised output stream. Thisprocessing results in a three or four byte clocking delay.

[0125] Packets with the secondary PID are routed by the PID identifier950 to a continuity count checker 954 which verifies sequence integrityfor this PID. Any errors are noted at 956, but specific handling oferrors is not relevant to understanding the present invention. Thepacket's continuity value is preserved for use in checking the sequenceof packets to follow. A corresponding continuity check 958 is done forpackets with the primary PID using the independent primary counter, andagain any errors are noted at 960.

[0126] The secondary packet is checked for a secondary flag at 962. ThisBoolean indicator is used to remember if a secondary packet has beenprocessed since the last clear packet. More than one secondary packetbetween clear packets is an error in this embodiment and is noted at964. Presence of a secondary packet is remembered by setting thesecondary flag at 966.

[0127] The continuity counter of the secondary packet is changed at 968to fit into the sequence of the clear packets. Data for thissubstitution comes from the value used to verify continuity of theprimary stream at 958. The revised packet is sent out from 968 andmerged into the revised stream forming the output stream.

[0128] After packets with primary PIDs have had their continuity checkedat 958, they are differentiated at 970 by the scrambling flags in theheader. If the packet is scrambled, the primary flag is queried at 974.This primary flag Boolean indicator is used to remember if a primaryencrypted packet has been processed since the last clear packet. Morethan one encrypted primary packet between clear packets is an error inthis embodiment and is noted at 976 before the packet is discarded at978. Presence of a encrypted primary packet is remembered by setting theprimary flag at 980. If there is no downstream consumer for the primaryencrypted packet, it can be discarded at 978. In some cases it may benecessary for the packet to continue on (in which case its continuitycounter can use the discarded secondary continuity value).

[0129] If the primary PID scramble test at 970 detects a clear packet,the state of the secondary and primary flags is tested at 984. Validconditions are neither set and both set, since encrypted packets shouldcome in matched pairs. A sequence of one without the other should benoted as an error at 988. However, the order of appearance isinconsequential in this embodiment. It should be noted that there may beother ways to flag a primary packet for deletion other than thescrambling bits in the transport header, e.g. the transport_prioritybit. Also, it is possible not to use any bits what-so-ever, e.g. usingthe primary packet's simple positional information, before or after thesecondary packet, as an indicator for replacement.

[0130] Clear packets with the primary PID then have their PID valuechanged at 992 to the secondary PID before being output in the revisedoutput stream. Alternatively, the secondary PID packets can be remappedto the primary PID value. The content can be decoded when the decoder isprovided with the correct PID for decoding the content (whether theprimary or secondary PID). Presence of a clear packet also clears theprimary and secondary Boolean flags.

[0131] In all the embodiments proposed, the secondary packet can beinserted adjoining the primary packet to be replaced even when a seriesof primary packets are tagged for replacement. However, in someinstances, it may facilitate headend partial scrambling if multipleencrypted packets can be inserted into the stream without theintervening secondary packets. In order to accommodate multipleconsecutive encrypted packets (such as with the M^(th) and N partialencryption method), the use of primary and secondary flags can bereplaced with a counter matching test function. Thus, in place ofelements 962, 964 and 966, a secondary encrypted packet counter can beincremented. In place of elements 970, 974, 976 and 980, a primaryencrypted packet counter can be incremented. Element 984 can be replacedwith a comparison of the primary and secondary encrypted packet countersto assure that the same number of encrypted packets are received in boththe primary and secondary paths. Instead of clearing flags at 992, thecounters are cleared. Using this variation, multiple encrypted packetsmay be consecutively received and the number received are compared tomonitor the integrity of the data stream. Other variations will occur tothose skilled in the art.

[0132] The function described above in connection with FIG. 17 can beintegrated into an A/V decoder chip that functions similar to that ofthe commercially available Broadcom series 70xx or 71xx decoder used incommercial set-top boxes. FIG. 18 illustrates a block diagram for such adecoder chip where the functions already provided in the commercial chipare essentially unchanged. Normally, commercial decoder chips expectthere to be a one-to-one correspondence between the PIDs and programcomponents (e.g., audio or video).

[0133] The decoder illustrated in FIG. 18 permits multiple PIDs to beprogrammed into the decoder via a connection to the STB centralprocessor so that both primary and secondary PIDs can be handled formain audio, main video and a secondary video used for picture-in-picture(PiP) functions. In this embodiment, the raw data stream is received bya Packet sorter 1002 that provides a function similar to that describedin connection with FIG. 17 above to demultiplex the stream of packetsbased upon PID. Preferably, the decoder of FIG. 18 carries out the PIDsorting function of 1002 using hard wired logic circuitry rather thanprogrammed software. Program guide and stream navigation information isoutput for use by an STB's main processor, for example. The packetsassociated with the main audio program are buffered in a FIFO 1006,decrypted in a decrypter 1010 and then buffered at 1014 for retrieval byan MPEG audio decoder 1018 as needed. Decoded MPEG audio is thenprovided as an output from the decoder.

[0134] In a similar manner, packets associated with the main videoprogram are buffered in a FIFO 1024, decrypted in a decrypter 1028 andthen buffered at 1032 for retrieval by an MPEG video decoder 1036 asneeded. Decoded MPEG video for the main channel is then provided to acompositer 1040 and then provided as an output from the decoder.Similarly, packets associated with picture-in-picture video are bufferedin a FIFO 1044, decrypted in a decrypter 1048 and then buffered at 1052for retrieval by an MPEG video decoder 1056 as needed. Decoded MPEGvideo for the picture-in-picture channel is then provided to thecompositer 1040 where it is combined with the main channel video andthen provided as a decoded video output from the decoder. Other packetsnot associated with the main or picture-in-picture channel arediscarded. Of course, other functions may be incorporated in the decoderchip or deleted without departing from embodiments of the presentinvention.

Conclusion

[0135] As previously mentioned, in order to thwart a persistent threatby hackers, several of the above partial encryption arrangements can becombined to further enhance security. For example, the critical packetencryption can be used in any combination with SI encryption, M^(th) anN, random encryption, time slice and other techniques to further enhancesecurity. In one embodiment, as many packets would be encrypted asbandwidth is available. The amount of encryption might depend on whetherthe content was a regular program or premium (such as a pay-per-view orVOD), whether it was an adult program or a regular movie, and thesecurity level that the various cable operators feel comfortableoperating. Those skilled in the art will appreciate that many othercombinations are possible to further enhance the security of theencryption without departing from the present invention.

[0136] The present invention, as described above in its variousembodiments, has been described in terms of a digital A/V system usingMPEG 2 coding. Thus, the various packet names and protocol specificallydiscussed is related the MPEG 2 coding and decoding. However, thoseskilled in the art will appreciate that the concepts disclosed andclaimed herein are not to be construed in such a limited scope. The sameor analogous techniques can be used in any digital cable system withoutlimitation to MPEG 2 protocols. Moreover, the present techniques can beused in any other suitable content delivery scenario including, but notlimited to, terrestrial broadcast based content delivery systems,Internet based content delivery, satellite based content deliverysystems such as, for example, the Digital Satellite Service (DSS) suchas that used in the DirecTV™ system, as well as package media (e.g. CDsand DVDs). These various alternatives are considered equivalent forpurposes of this document, and the exemplary MPEG 2 cable embodimentshould be considered to be an exemplary embodiment presented forillustrative purposes.

[0137] In addition, the present invention has been described in terms ofdecoding partially encrypted television programs using a televisionset-top box. However, the present decoding mechanism can equally beimplemented within a television receiver without need for an STB, ormusic player such as an MP3 player. Such embodiments are consideredequivalent.

[0138] Also, while the present invention has been described in terms ofthe use of the encryption techniques described to provide a mechanismfor dual partial encryption of a television program, these partialencryption techniques could be used as a single encryption technique orfor multiple encryption under more than two encryption systems withoutlimitation. More than two encryption systems would be accommodated withadditional duplicated packets that are encrypted. Alternatively, theencryption key for one of the duplicated packets may be shared amongstthe multiple encryption systems. Additionally, although specificallydisclosed for the purpose of encryption of television programming, thepresent inventions can be utilized for single or dual encryption ofother content including, but not limited to content for download overthe Internet or other network, music content, packaged media content aswell as other types of information content. Such content may be playedon any number of playback devices including but not limited to personaldigital assistants (PDAs), personal computers, personal music players,audio systems, audio/video systems, etc. without departing from thepresent invention.

[0139] Those skilled in the art will recognize that the presentinvention has been described in terms of exemplary embodiments that canbe realized by use of a programmed processor. However, the inventionshould not be so limited, since the present invention could beimplemented using hardware component equivalents such as special purposehardware and/or dedicated processors which are equivalents to theinvention as described and claimed. Similarly, general purposecomputers, microprocessor based computers, micro-controllers, opticalcomputers, analog computers, dedicated processors and/or dedicated hardwired logic may be used to construct alternative equivalent embodimentsof the present invention.

[0140] Those skilled in the art will appreciate that the program stepsand associated data used to implement the embodiments described abovecan be implemented using disc storage as well as other forms of storagesuch as for example Read Only Memory (ROM) devices, Random Access Memory(RAM) devices; optical storage elements, magnetic storage elements,magneto-optical storage elements, flash memory, core memory and/or otherequivalent storage technologies without departing from the presentinvention. Such alternative storage devices should be consideredequivalents.

[0141] The present invention, as described in embodiments herein, can beimplemented using a programmed processor executing programminginstructions that are broadly described above in flow chart form thatcan be stored on any suitable electronic storage medium or transmittedover any suitable electronic communication medium. However, thoseskilled in the art will appreciate that the processes described abovecan be implemented in any number of variations and in many suitableprogramming languages without departing from the present invention. Forexample, the order of certain operations carried out can often bevaried, additional operations can be added or operations can be deletedwithout departing from the invention. Error trapping can be added and/orenhanced and variations can be made in user interface and informationpresentation without departing from the present invention. Suchvariations are contemplated and considered equivalent.

[0142] While the invention has been described in conjunction withspecific embodiments, it is evident that many alternatives,modifications, permutations and variations will become apparent to thoseskilled in the art in light of the foregoing description. Accordingly,it is intended that the present invention embrace all such alternatives,modifications and variations as fall within the scope of the appendedclaims.

What is claimed is:
 1. A method of encrypting an unencrypted televisionprogram, comprising: sampling the unencrypted television program at aspecified time interval; for each sample: encrypting the sampleaccording to a first encryption method to create a first encryptedsample; and encrypting the sample according to a second encryptionmethod to create a second encrypted sample.
 2. The method according toclaim 1, further comprising, combining the first and second encryptedsamples with unsampled portions of the unencrypted television program toproduce partially dual encrypted television programs.
 3. The methodaccording to claim 2, further comprising distributing the partially dualencrypted television program over a communication medium.
 4. The methodaccording to claim 2, further comprising assigning a of pluralityprimary packet identifiers (PID) to data packets containing unencryptedportions of the television program, the primary packet identifiersassociating the unencrypted portion with the television program.
 5. Themethod according to claim 2, further comprising assigning a plurality ofprimary packet identifiers (PID) to data packets containing firstencrypted samples of the television program, the primary packetidentifiers associating the first encrypted samples with the televisionprogram.
 6. The method according to claim 2, further comprisingassigning a plurality of secondary packet identifiers (PID) to datapackets containing second encrypted samples of the television program,the secondary packet identifiers associating the second encryptedsamples with the television program.
 7. The method according to claim 2,further comprising: assigning a plurality of primary packet identifiers(PID) to data packets containing unencrypted portions of the televisionprogram, the primary packet identifiers associating the unencryptedportions with the television program; assigning the plurality of primarypacket identifiers to data packets containing first encrypted samples ofthe television program, the primary packet identifiers associating thefirst encrypted samples with the television program; and assigning aplurality of secondary packet identifiers to data packets containingsecond encrypted samples of the television program, the secondary packetidentifiers associating the second encrypted samples with the televisionprogram.
 8. The method according to claim 7, further comprisingtransmitting system information to identify the primary and secondaryPID associated with the television program.
 9. The method according toclaim 1, wherein the sample comprises a data associated with a frame ofvideo.
 10. The method according to claim 1, wherein the sample comprisesat least one packet of data.
 11. The method according to claim 1,wherein the specified time interval comprises a randomly occurring timeinterval.
 12. The method according to claim 1, further comprisingdistributing at least one of the first and second encrypted samplesseparately from unsampled portions of the unencrypted televisionprogram.
 13. An electronic storage medium storing instructions which,when executed on a programmed processor, carry out the method accordingto claim
 1. 14. An electronic transmission medium carrying an encryptedtelevision signal encrypted by the method according to claim
 1. 15. Anencrypted television signal, comprising: a first encrypted sample of thetelevision signal, the first encrypted sample comprising a first sampleencrypted under a first encryption method; a second encrypted sample ofthe television signal, the second encrypted sample comprising the firstsample encrypted under a second encryption method; and an unencryptedportion.
 16. The encrypted television signal according to claim 13,wherein the television signal comprises a digital television signal, andwherein the first encrypted samples and second encrypted samplescomprise first encrypted packets and second encrypted packets, and theunencrypted portion comprises unencrypted packets.
 17. The encryptedtelevision signal according to claim 16, wherein the digital televisionsignal complies with an MPEG standard, and wherein the first encryptedpackets are identified by a first packet identifier, and wherein thesecond encrypted packets are identified by a second packet identifier.18. The encrypted television signal according to claim 14, wherein thedigital television signal complies with an MPEG standard, and whereinthe unencrypted packets are identified by a first packet identifier, andwherein the first encrypted packets are identified by the first packetidentifier, and wherein the second encrypted packets are identified by asecond packet identifier.
 19. A method of encrypting an unencryptedtelevision program, comprising: sampling the unencrypted televisionprogram at a specified time interval; and for each sample, encryptingthe sample according to a first encryption method to create a firstencrypted sample for the television program.
 20. The method according toclaim 19, further comprising, combining the first encrypted samples withthe unsampled portions of the unencrypted television program to producea partially encrypted television program.
 21. The method according toclaim 20, further comprising distributing the partially encryptedtelevision program over a cable television system.
 22. The methodaccording to claim 20, further comprising assigning a packet identifier(PID) to data packets containing unencrypted portions of the televisionprogram, the packet identifier associating the unencrypted portion witha particular television program.
 23. The method according to claim 20,further comprising assigning a packet identifier (PID) to data packetscontaining first encrypted samples of the television program, the packetidentifier associating the first encrypted samples with a particulartelevision program.
 24. The method according to claim 20, furthercomprising assigning a secondary packet identifier (PID) to data packetscontaining first encrypted sample of the television program, thesecondary packet identifier associating the first encrypted samples witha particular television program.
 25. The method according to claim 20,further comprising assigning a packet identifier (PID) to data packetscontaining first encrypted samples and unencrypted portions of thetelevision program, the packet identifier associating the firstencrypted samples and the unencrypted portions with a particulartelevision program.
 26. The method according to claim 20, furthercomprising assigning a primary packet identifier (PID) to data packetscontaining unencrypted portions of the television program, the packetidentifier associating the unencrypted portions with a particulartelevision program; and assigning a secondary packet identifier (PID) todata packets containing encrypted samples of the television program, thesecondary packet identifier associating the encrypted samples with theparticular television program
 27. The method according to claim 20,wherein the sample comprises a data associated with a frame of video.28. The method according to claim 19, wherein the sample comprises atleast one packet of data.
 29. An electronic storage medium storinginstructions which, when executed on a programmed processor, carry outthe method according to claim
 19. 30. An electronic transmission mediumcarrying an encrypted television program encrypted by the methodaccording to claim
 19. 31. A method of encrypting an unencryptedtelevision program, comprising: encrypting N periods out of every Mperiods of the television program according to a first encryptionmethod, where M is greater than N; and encrypting the N periods of thetelevision program according to a second encryption method.
 32. Themethod according to claim 31, further comprising, combining the firstand second encrypted periods with unencrypted periods to produce apartially dual encrypted television program.
 33. The method according toclaim 32, further comprising distributing the partially dual encryptedtelevision program over a cable television system.
 34. The methodaccording to claim 32, further comprising assigning a primary packetidentifier (PID) to unencrypted periods of the television program. 35.The method according to claim 32, further comprising assigning a primarypacket identifier (PID) to periods encrypted under the first encryptionmethod.
 36. The method according to claim 32, further comprisingassigning a secondary packet identifier (PID) to periods encrypted underthe second encryption method.
 37. The method according to claim 32,further comprising: assigning a primary packet identifier (PID) tounencrypted periods of the television program; assigning a primarypacket identifier(PID)to periods encrypted under the first encryptionmethod; and assigning a secondary packet identifier (PID) to periodsencrypted under the second encryption method.
 38. The method accordingto claim 37, further comprising transmitting system information toidentify the primary and secondary PID associated with the televisionprogram.
 39. The method according to claim 32, wherein the periodcomprises data associated with a frame of video.
 40. The methodaccording to claim 32, wherein the period comprises at least one packetof data.
 41. The method according to claim 31, wherein the N periods andM periods are selected randomly.
 42. An electronic storage mediumstoring instructions which, when executed on a programmed processor,carry out the method according to claim
 32. 43. An electronictransmission medium carrying an encrypted television program encryptedby the method according to claim
 32. 44. A partially encryptedtelevision signal, comprising: a first portion of the television signal,the portion being encrypted under a first encryption method and under asecond encryption method; and an unencrypted portion, wherein for everyM periods of the television signal, N periods are encrypted.
 45. Thepartially encrypted television signal according to claim 44, wherein thetelevision signal is a digital television signal, and wherein the periodcomprises a packet.
 46. The partially encrypted television signalaccording to claim 44, wherein the period comprises a video frame. 47.The partially encrypted television signal according to claim 44, whereinthe digital television signal complies with an MPEG standard, andwherein the first encrypted portion is identified by a first packetidentifier, and wherein the second encrypted portion is identified by asecond packet identifier.
 48. The partially encrypted television signalaccording to claim 44, wherein the digital television signal complieswith an MPEG standard, and wherein the unencrypted portion is identifiedby a first packet identifier, and wherein the encrypted portionencrypted under the first encryption method is identified by the firstpacket identifier, and wherein the encrypted portion encrypted under thesecond encryption method is identified by a second packet identifier.49. A method of encrypting an unencrypted television program,comprising: encrypting N portions of the television program out of everyM portions of the television program according to a first encryptionmethod; and leaving a remainder of the television program unencrypted,wherein the combination of the encrypted portion and the remaindercomprises a partially encrypted television program.
 50. The methodaccording to claim 49, further comprising distributing the partiallyencrypted television program over one of the following: a terrestrialbroadcast system, a cable television system and a satellite televisionsystem.
 51. The method according to claim 49, further comprisingassigning a packet identifier (PID) to data packets containing theencrypted portions and the remainder of the television program, thepacket identifier associating the encrypted portion and the remainderwith the television program.
 52. The method according to claim 49,further comprising assigning a primary packet identifier (PID) to datapackets containing the encrypted portions and the remainder of thetelevision program, the packet identifier associating the encryptedportion and the remainder with the television program; and assigning asecondary packet identifier (PID) to data packets containing theencrypted portions of each television program, the secondary packetidentifier associating the first encrypted samples with the televisionprogram.
 53. The method according to claim 49, wherein the N portionscomprise N frames of video.
 54. An electronic storage medium storinginstructions which, when executed on a programmed processor, carry outthe method according to claim
 49. 55. An electronic transmission mediumcarrying an encrypted television program encrypted by the methodaccording to claim
 49. 56. A method of processing a television signal,comprising: receiving a television signal comprising: a first encryptedsample of the television program, the sample being encrypted under afirst encryption method, a second encrypted sample of the televisionprogram, the sample being encrypted under a second encryption method,and an unencrypted portion; and decrypting the first encrypted sample toproduce a decrypted sample.
 57. The method according to claim 56,further comprising decoding the unencrypted portion and the decryptedsample.
 58. The method according to claim 56, wherein the televisionsignal is a digital television signal, and wherein the first encryptedsamples and second encrypted samples comprise first encrypted packetsand second encrypted packets, and the unencrypted portion comprisesunencrypted packets.
 59. The method according to claim 58, wherein thedigital television signal complies with an MPEG standard, and whereinthe first encrypted packets are identified by a first packet identifier,and wherein the second encrypted packets are identified by a secondpacket identifier.
 60. The method according to claim 59, wherein thedigital television signal complies with an MPEG standard, and whereinthe unencrypted packets are identified by a first packet identifier, andwherein the first encrypted packets are identified by the first packetidentifier, and wherein the second encrypted packets are identified by asecond packet identifier.
 61. The method according to claim 56, carriedout in a television device.
 62. The method according to claim 56,carried out in a television set-top box.
 63. The method according toclaim 56, carried out in an integrated circuit.
 64. The method accordingto claim 56, carried out in one of an application specific integratedcircuit and a field programmable gate array.
 65. A method of processingdigital content, comprising: receiving a signal containing the digitalcontent, the digital content comprising: a first encrypted sample thedigital content, the sample being encrypted under a first encryptionmethod, a second encrypted sample of the digital content, the samplebeing encrypted under a second encryption method, and an unencryptedportion; and decrypting the first encrypted sample to produce adecrypted sample.
 66. The method according to claim 65, furthercomprising decoding the unencrypted portion and the decrypted sample.67. The method according to claim 65, carried out in an integratedcircuit.
 68. The method according to claim 65, carried out in one of atelevision device, a content player, a PDA and a music player.
 69. Amethod of decoding a television program, comprising: receiving atelevision signal having N periods out of every M periods of thetelevision program encrypted according to a first encryption method,where M is greater than N, and wherein the same N periods of thetelevision program encrypted according to a second encryption method,wherein M-N periods are unencrypted; decrypting the N periods accordingto a first decryption method to produce decrypted periods; discardingthe N periods encrypted according to the second encryption method; anddecoding the decrypted periods and the unencrypted periods.
 70. Themethod according to claim 69, further comprising filtering out the Nperiods encrypted according to the second encryption method.
 71. Themethod according to claim 70, wherein the filtering is carried out byfiltering on a packet identifier (PID) associated with data packets. 72.The method according to claim 69, wherein the period comprises dataassociated with a frame of video.
 73. The method according to claim 69,wherein the period comprises at least one packet of data.
 74. The methodaccording to claim 69, wherein the period comprises data associated witha frame of audio.
 75. An electronic storage medium storing instructionswhich, when executed on a programmed processor, carry out the methodaccording to claim
 69. 76. The method according to claim 69, carried outin a television device.
 77. The method according to claim 69, carriedout in a television set-top box.
 78. The method according to claim 69,carried out in an integrated circuit.
 79. The method according to claim69, carried out in one of an application specific integrated circuit anda field programmable gate array.
 80. A television set-top box fordecoding a television signal, comprising: a receiver that receives atelevision signal having: a first encrypted sample of the televisionprogram, the sample being encrypted under a first encryption method, asecond encrypted sample of the television program, the sample beingencrypted under a second encryption method, and an unencrypted portion;a decrypter that decrypts the first encrypted sample to produce adecrypted sample; and a decoder that decodes the unencrypted portion andthe decrypted sample.
 81. The apparatus according to claim 80, whereinthe television signal is a digital television signal, and wherein thefirst encrypted samples and second encrypted samples comprise firstencrypted packets and second encrypted packets, and the unencryptedportion comprises unencrypted packets.
 82. The apparatus according toclaim 81, wherein the digital television signal complies with an MPEGstandard, and wherein the first encrypted packets are identified by afirst packet identifier, and wherein the second encrypted packets areidentified by a second packet identifier.
 83. The apparatus according toclaim 81, wherein the digital television signal complies with an MPEGstandard, and wherein the unencrypted packets are identified by a firstpacket identifier, and wherein the first encrypted packets areidentified by the first packet identifier, and wherein the secondencrypted packets are identified by a second packet identifier.
 84. Atelevision set-top box for decoding a television program, comprising: areceiver receiving a television program having N periods out of every Mperiods of the television program encrypted according to a firstencryption method, where M is greater than N, and wherein the same Nperiods of the television program encrypted according to a secondencryption method, wherein M-N periods are unencrypted; a decrypter thatdecrypts the N periods according to a first decryption method to producedecrypted periods; filtering means for discarding the N periodsencrypted according to the second encryption method; and a decoder thatdecodes the decrypted periods and the unencrypted periods.
 85. Themethod according to claim 84, further comprising filtering out the Nperiods encrypted according to the second encryption method.
 86. Themethod according to claim 85, wherein the filtering is carried out byfiltering on a packet identifier (PID) associated with data packets. 87.The method according to claim 84, wherein the period comprises dataassociated with a frame of video.
 88. The method according to claim 84,wherein the period comprises at least one packet of data.
 89. Anelectronic storage medium storing instructions which, when executed on aprogrammed processor, carry out the method according to claim
 84. 90. Amethod of encrypting a plurality of unencrypted television programs,comprising: selecting a video frame from each unencrypted televisionprogram at a specified time interval; encrypting the frame according toa first encryption method to create a first encrypted frame for eachtelevision program; and encrypting the frame according to a secondencryption method to create a second encrypted frame for each televisionprogram.
 91. The method according to claim 90, further comprising,combining the first and second encrypted frames with unencrypted framesof the unencrypted television programs to produce partially dualencrypted television programs.
 92. The method according to claim 91,further comprising distributing the partially dual encrypted televisionprograms over one of a cable television system, a terrestrial broadcastsystem, and a satellite system.
 93. The method according to claim 91,further comprising assigning a plurality primary packet identifiers(PID) to data packets containing unencrypted portions of each televisionprogram, the primary packet identifiers associating the unencryptedportions with each particular television program.
 94. The methodaccording to claim 91, further comprising assigning a plurality primarypacket identifiers (PID) to data packets containing first encryptedframes of each television program, the primary packet identifiersassociating the first encrypted frames with each particular televisionprogram.
 95. The method according to claim 91, further comprisingassigning a plurality of secondary packet identifiers (PID) to datapackets containing second encrypted frames of each television program,the secondary packet identifiers associating the second encrypted frameswith a particular television program.
 96. The method according to claim91, further comprising: assigning a plurality primary packet identifiers(PID) to data packets containing unencrypted portions of each televisionprogram, the primary packet identifiers associating the unencryptedportions with each particular television program; assigning theplurality of primary packet identifiers to data packets containing firstencrypted frames of each television program, the primary packetidentifiers associating the first encrypted frames with each particulartelevision program; and assigning a plurality of secondary packetidentifiers to data packets containing second encrypted frames of eachtelevision program, the secondary packet identifiers associating thesecond encrypted samples with a particular television program.
 97. Themethod according to claim 96, further comprising transmitting systeminformation to identify the primary and secondary packet identifiersassociated with each television program.
 98. The method according toclaim 90, wherein the specified time interval is selected at random. 99.An electronic storage medium storing instructions which, when executedon a programmed processor, carry out the method according to claim 90.100. An electronic transmission medium carrying an encrypted televisionsignal encrypted by the method according to claim
 90. 101. A method ofencrypting an unencrypted television program, comprising: selecting aframe of the unencrypted television program at a specified timeinterval; and encrypting the frame according to a first encryptionmethod to create a first encrypted sample for the television program.102. The method according to claim 101, further comprising, combiningthe first encrypted frame with unencrypted portions of the televisionprogram to produce a partially encrypted television program.
 103. Themethod according to claim 102, further comprising distributing thepartially encrypted television program over a cable television system.104. The method according to claim 102, further comprising assigning apacket identifier (PID) to data packets containing unencrypted portionsof the television program, the packet identifier associating theunencrypted portion with a particular television program.
 105. Themethod according to claim 102, further comprising assigning a packetidentifier (PID) to data packets containing first encrypted frames ofthe television program, the packet identifier associating the firstencrypted frames with a particular television program.
 106. The methodaccording to claim 102, further comprising assigning a secondary packetidentifier (PID) to data packets containing first encrypted frames ofthe television program, the secondary packet identifier associating thefirst encrypted samples with a particular television program.
 107. Themethod according to claim 102, further comprising assigning a packetidentifier (PID) to data packets containing first encrypted frames andunencrypted portions of the television program, the packet identifierassociating the first encrypted frames and the unencrypted portions witha particular television program.
 108. The method according to claim 102,further comprising assigning a primary packet identifier (PID) to datapackets containing unencrypted portions of the television program, thepacket identifier associating the unencrypted portions with a particulartelevision program; and assigning a secondary packet identifier (PID) todata packets containing encrypted frames of the television program, thesecondary packet identifier associating the encrypted frames with theparticular television program
 109. An electronic storage medium storinginstructions which, when executed on a programmed processor, carry outthe method according to claim
 101. 110. An electronic transmissionmedium carrying an encrypted television program encrypted by the methodaccording to claim 101.